fix(retrieval): give reuse a slot, and let experience reach the write path

Two mirror-image scoping mistakes, neither deliberate (#2246).

AUTO-INJECT let every kind compete on raw cosine. That is fatal rather than
merely imperfect here, because Scribe's project records are ABOUT software
work: a task titled "surface snippets before the agent writes code" is a
near-perfect lexical match for "write a function…" while answering none of it.
Measured live, a prompt asking for a helper returned three records about
BUILDING the retrieval system and zero snippets. Snippets are ~0.5% of the
corpus, and the ratio worsens as the project record grows — which is the
direction Scribe is meant to grow, so no threshold tuning fixes it.

Now the best snippet or process takes the LAST slot when none won on score.
Deliberately NOT held to the margin band: that band measures distance from the
top overall score, and the top score is the very thing snippets lose to. It
still must clear the configured threshold, so a weak snippet cannot buy the
slot — silence stays the default. Skipped entirely when reuse already won,
so the fix is invisible in the case it isn't needed.

WRITE-PATH was snippets-only — the same mistake inverted. An issue recording
"we tried this and it deadlocked" could never reach the moment that code was
about to be written, though it is arguably the better prior art: it says what
NOT to do. Widened to snippets plus recorded experience.

That needed a filter the search layer couldn't express. "Experience" is issues
plus dev-logs, which differ on is_task, so neither note_type nor is_task alone
covers it. semantic_search_notes gains task_kind, which restricts TASKS to the
given kinds while leaving non-task notes untouched — so note_type=("snippet",
"note") + task_kind="issue" yields snippets, fixed problems and durable notes,
without the open to-do list. note_type now accepts a sequence too.

Non-snippet hits are labelled with their kind, because an unlabelled issue on
that menu reads as "here is code to reuse", the opposite of what it says.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

tests/test_services_plugin_context.py

@@ -295,3 +295,174 @@ async def test_build_session_context_caps_length():
 
     assert len(out["context"]) <= 9000 + 60  # cap + truncation note
     assert "truncated" in out["context"]
+
+
+# --- the reuse slot (#2246) --------------------------------------------------
+
+_CFG = {"enabled": True, "threshold": 0.55, "top_k": 3}
+
+
+async def _autoinject(main_hits, reuse_hits, cfg=None):
+    """Run build_autoinject_hint with the two semantic queries stubbed in order:
+    the unscoped pool first, then the reserved reuse query."""
+    calls: list[dict] = []
+
+    async def fake_search(*_a, **kw):
+        calls.append(kw)
+        return reuse_hits if kw.get("note_type") else main_hits
+
+    with patch("scribe.services.plugin_context.get_autoinject_config",
+               AsyncMock(return_value=dict(cfg or _CFG))), \
+         patch("scribe.services.plugin_context.semantic_search_notes",
+               AsyncMock(side_effect=fake_search)), \
+         patch("scribe.services.plugin_context.record_retrieval", MagicMock()), \
+         patch("scribe.services.plugin_context.record_surfaced", MagicMock()), \
+         patch("scribe.services.plugin_context.owner_names_for",
+               AsyncMock(return_value={})):
+        from scribe.services.plugin_context import build_autoinject_hint
+        out = await build_autoinject_hint(1, "write a debounce helper")
+    return out, calls
+
+
+@pytest.mark.asyncio
+async def test_a_snippet_takes_the_last_slot_when_none_won_on_score():
+    """The measured failure: a prompt asking for a helper returned three project
+    records ABOUT building the retrieval system, and zero snippets. Scribe's own
+    records are about software work, so they share vocabulary with any coding
+    prompt while answering none of them."""
+    main = [(0.66, _note(1, "Step 3: title-first auto-inject")),
+            (0.65, _note(2, "Task-reminder dedup query crashes", is_task=True)),
+            (0.64, _note(3, "Drafter hardening · write-path trigger", is_task=True))]
+    reuse = [(0.58, _note(9, "debounce — collapse rapid calls", note_type="snippet"))]
+
+    out, calls = await _autoinject(main, reuse)
+
+    assert out["note_ids"] == [1, 2, 9]        # last slot displaced, not the first
+    assert "#9" in out["context"] and "[snippet]" in out["context"]
+    # The reserved query is scoped to the reuse kinds and asks for exactly one.
+    assert calls[1]["note_type"] == ("snippet", "process")
+    assert calls[1]["limit"] == 1
+
+
+@pytest.mark.asyncio
+async def test_the_reserved_query_is_skipped_when_a_snippet_already_won():
+    """No second query, and no slot spent twice, when ranking already did the
+    right thing — the fix must be invisible in the case it isn't needed."""
+    main = [(0.81, _note(9, "debounce helper", note_type="snippet")),
+            (0.80, _note(1, "some task", is_task=True))]
+
+    out, calls = await _autoinject(main, [])
+
+    assert len(calls) == 1                      # reserved query never ran
+    assert out["note_ids"] == [9, 1]
+
+
+@pytest.mark.asyncio
+async def test_a_weak_snippet_does_not_buy_the_slot():
+    """The reserved hit skips the MARGIN band — that band is what snippets lose
+    to — but never the threshold. Silence stays the default; a slot spent on an
+    irrelevant snippet is how a menu teaches people to ignore it."""
+    main = [(0.66, _note(1, "a task", is_task=True))]
+
+    out, calls = await _autoinject(main, [])     # threshold returned nothing
+
+    assert out["note_ids"] == [1]
+    assert len(calls) == 2                       # it asked, and got nothing
+
+
+@pytest.mark.asyncio
+async def test_the_reserved_hit_is_not_held_to_the_margin_band():
+    """0.58 is 0.08 below the top hit. Under the band it would survive; the point
+    is that it must survive even when it wouldn't — a snippet losing to a
+    same-vocabulary project record by a wide margin is the whole bug."""
+    main = [(0.90, _note(1, "a task", is_task=True))]
+    reuse = [(0.58, _note(9, "debounce", note_type="snippet"))]
+
+    out, _calls = await _autoinject(main, reuse)
+
+    assert 9 in out["note_ids"]
+
+
+@pytest.mark.asyncio
+async def test_a_process_counts_as_reuse_too():
+    """A stored process answers 'how do we do X here' the same way a snippet
+    answers 'what do we already have' — both lose to the same project records."""
+    main = [(0.70, _note(1, "a task", is_task=True))]
+    reuse = [(0.60, _note(8, "DRY pass process", note_type="process"))]
+
+    out, _ = await _autoinject(main, reuse)
+    assert 8 in out["note_ids"]
+
+    # …and one already on the menu suppresses the reserved query.
+    out2, calls2 = await _autoinject(
+        [(0.70, _note(8, "DRY pass process", note_type="process"))], [])
+    assert len(calls2) == 1
+
+
+# --- write-path widened beyond snippets (#2246, the mirror half) -------------
+
+@pytest.mark.asyncio
+async def test_write_path_semantic_arm_asks_for_experience_not_just_snippets():
+    """The inverse of auto-inject's mistake. This arm was snippets-only, so an
+    issue saying "we tried this and it deadlocked" could never reach the moment
+    that code was about to be written — arguably the better prior art, because
+    it says what NOT to do."""
+    from scribe.services import plugin_context as pc
+
+    hits = [(0.72, _note(9, "debounce helper", note_type="snippet")),
+            (0.70, _note(7, "Debounce dropped the trailing call", is_task=True,
+                         task_kind="issue"))]
+    search = AsyncMock(return_value=hits)
+    rec = MagicMock()
+    with patch.object(pc, "get_writepath_config",
+                      AsyncMock(return_value={"enabled": True, "threshold": 0.6,
+                                              "top_k": 3})), \
+         patch.object(pc.snippets_svc, "list_snippets",
+                      AsyncMock(return_value=([], 0))), \
+         patch.object(pc, "semantic_search_notes", search), \
+         patch.object(pc, "record_retrieval", rec), \
+         patch.object(pc, "record_surfaced", MagicMock()), \
+         patch.object(pc, "owner_names_for", AsyncMock(return_value={})), \
+         patch.object(pc, "concept_query", MagicMock(return_value="debounce a callback")):
+        out = await pc.build_write_path_hint(
+            1, "src/utils/debounce.ts", code="x" * 400,
+        )
+
+    kw = search.await_args.kwargs
+    assert kw["note_type"] == ("snippet", "note")
+    # An open to-do resembling the code answers nothing; an ISSUE carries a root
+    # cause and a NOTE carries durable knowledge. Only the todo is excluded.
+    assert kw["task_kind"] == "issue"
+    # Telemetry must not claim this was a notes-only retrieval any more.
+    assert rec.call_args.kwargs["is_task"] is None
+
+
+@pytest.mark.asyncio
+async def test_write_path_labels_a_non_snippet_hit_with_its_kind():
+    """An unlabelled issue on this menu reads as "here is code to reuse", which
+    is the opposite of what it says. A snippet stays unlabelled — it is the
+    menu's default and the header's default reading."""
+    from scribe.services import plugin_context as pc
+
+    hits = [(0.72, _note(9, "debounce helper", note_type="snippet")),
+            (0.71, _note(7, "Debounce dropped the trailing call", is_task=True,
+                         task_kind="issue"))]
+    with patch.object(pc, "get_writepath_config",
+                      AsyncMock(return_value={"enabled": True, "threshold": 0.6,
+                                              "top_k": 3})), \
+         patch.object(pc.snippets_svc, "list_snippets",
+                      AsyncMock(return_value=([], 0))), \
+         patch.object(pc, "semantic_search_notes", AsyncMock(return_value=hits)), \
+         patch.object(pc, "record_retrieval", MagicMock()), \
+         patch.object(pc, "record_surfaced", MagicMock()), \
+         patch.object(pc, "owner_names_for", AsyncMock(return_value={})), \
+         patch.object(pc, "concept_query", MagicMock(return_value="debounce a callback")):
+        out = await pc.build_write_path_hint(
+            1, "src/utils/debounce.ts", code="x" * 400,
+        )
+
+    ctx = out["context"]
+    assert "· issue]" in ctx                 # the issue says what it is
+    assert '[similar 0.72] "debounce helper"' in ctx   # the snippet does not
+    # The header now names the right opener for each kind.
+    assert "get_task(id)" in ctx and "get_snippet(id)" in ctx