feat(retrieval): a System's charter becomes an answer, not just a filter (#4251)

Step 2 of #4251. A System's `description` is a charter — several hundred words
saying what belongs in that area and what does not — and it is the answer to
"which part of this codebase does X live in". There was no semantic path to
one: `list_systems` enumerates, and `search(system_id=…)` uses a System as a
FILTER over notes. So a System could narrow a search and could never be the
answer to one, and an agent asking where a record belonged had to read every
charter or guess.

ITS OWN SEARCH, not a `content_type` over notes, for the reason note 3163
gives about milestones: the row could be shared, the search cannot. A charter
competing with the whole note corpus for one top-k is outranked by the records
filed under it — the right answer crowded out by its own contents — and "where
does this belong?" is a different question from "what prior art is there?",
which a caller asking one should not have to read past answers to.

So `system_embeddings` (0107) joins note_, rule_ and milestone_embeddings as
the fourth sibling, with `system_document`, `upsert_system_embedding`,
`semantic_search_systems`, a startup backfill and `search(content_type=
"system")`. Scoped like milestones: with a project_id, that project's Systems
if the caller can read the project (rule 78); without one, the caller's own.
Archived Systems are excluded — an archived area is one the operator has said
is no longer where things go, which is exactly the question being asked.

`system_document` is the plainest of the four shapes on purpose. A charter is
already written as the thing this search has to match, in the words someone
asking would use — so there is no trigger to synthesise as `rule_document`
must, and no second record to gather as `task_document` must. The stored
charter IS the sharp document, the way a snippet's is. `color`, `status` and
`order_index` stay out: presentation and bookkeeping, and a vector carrying
them would be answering a question nobody asks of a charter.

The search publishes `report["best_chunk"]` from the start rather than being
retrofitted, which is what #4251 asked of any fourth search. It matters more
here than anywhere: a charter runs long and a result shows its NAME, so a match
on the paragraph that actually decides where a record belongs would otherwise
be previewed by two words that cannot say.

The id that comes back is the one `system_id`, `system_ids` and
`list_system_records` already take, so the answer to "where does this belong?"
is directly usable as "show me what is there" and as "file it here".

`embed_system` sits beside `notes.embed_note` at the service for #2056's
reason — every door gets it by construction. Not called on delete: that is a
soft delete and the search joins through `System`, so the vectors are already
unreachable, and leaving them means a restore is findable again immediately.
`system_embeddings` is declared in backup's `_NOT_INCLUDED` as derived, beside
its three siblings.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

tests/test_mcp_tool_search.py

@@ -1,7 +1,7 @@
 """search tool — proves the tool pattern (context + service call + dict shape).
 
 Service call is mocked; no DB needed."""
-from unittest.mock import AsyncMock, patch
+from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
@@ -223,7 +223,8 @@ async def test_the_docstring_names_every_kind_the_tool_accepts():
     doc = search_tool.__doc__ or ""
     for facet in FACET_TYPES:
         assert f"'{facet}'" in doc, f"{facet} is accepted but never documented"
-    assert "'rule'" in doc and "'milestone'" in doc and "'all'" in doc
+    for own in ("'rule'", "'milestone'", "'system'", "'all'"):
+        assert own in doc, f"{own} dispatches somewhere and is never documented"
 
 
 @pytest.mark.asyncio
@@ -329,3 +330,80 @@ async def test_milestone_search_is_its_own_shape_and_scopes_to_the_project():
         "status": "active", "project_id": 30, "total": 0, "completed": 0,
         "similarity": 0.81,
     }]
+
+
+# --- systems: the charter as an answer, not a filter (#4251) -----------------
+
+
+@pytest.mark.asyncio
+async def test_system_search_is_its_own_shape_and_carries_the_matched_passage():
+    """A System's charter runs to several hundred words and a result shows its
+    NAME — so a match on the paragraph that actually decides where a record
+    belongs would be previewed by two words that cannot. The passage comes
+    along, marked as the passage (#4243)."""
+    _user_id_ctx.set(7)
+    charter = (
+        "Opening sentence about the area in general. "
+        + "x" * 400
+        + " Retrieval telemetry and the floors it judges belong here."
+    )
+    fake = MagicMock(id=3, project_id=2, description=charter)
+    # `name` is MagicMock's own constructor kwarg — passed in, it names the
+    # mock and leaves `.name` a mock object, which is note #2833's whole point.
+    fake.name = "Retrieval & recall"
+
+    async def _system_search(uid, q, **kwargs):
+        kwargs["report"]["best_chunk"] = {
+            3: {"index": 2, "text": "Retrieval telemetry and the floors it judges belong here."}
+        }
+        return [(0.81, fake)]
+
+    with patch("scribe.mcp.tools.search.semantic_search_systems", _system_search):
+        out = await search(q="where does retrieval telemetry go?",
+                           content_type="system", project_id=2)
+
+    assert out["total"] == 1
+    row = out["results"][0]
+    assert row["id"] == 3
+    assert row["name"] == "Retrieval & recall"
+    assert row["project_id"] == 2
+    assert row["similarity"] == 0.81
+    # The passage that won, not the charter's opening — and SAID to be that.
+    assert row["matched"] == (
+        "Retrieval telemetry and the floors it judges belong here."
+    )
+    assert row["matched_is"] == "matched_passage"
+    assert row["body_length"] == len(charter)
+    assert "read_full" in row, "a 56-character span of a 500-character charter"
+    # The charter itself does not ride along: get_system reads it.
+    assert "description" not in row
+
+
+@pytest.mark.asyncio
+async def test_system_search_scopes_to_the_project_it_was_given():
+    """Systems are per-project and a charter from another project is not an
+    answer to "where does this belong here?"."""
+    _user_id_ctx.set(7)
+    mock = AsyncMock(return_value=[])
+    with patch("scribe.mcp.tools.search.semantic_search_systems", mock):
+        await search(q="x", content_type="system", project_id=30)
+    assert mock.call_args.kwargs["project_id"] == 30
+
+    mock.reset_mock()
+    with patch("scribe.mcp.tools.search.semantic_search_systems", mock):
+        await search(q="x", content_type="system")
+    assert mock.call_args.kwargs["project_id"] is None
+
+
+@pytest.mark.asyncio
+async def test_a_system_search_does_not_go_through_the_note_search():
+    """Its own search, not a note_type. A charter competing with the whole note
+    corpus for one top-k is outranked by the records filed under it."""
+    _user_id_ctx.set(7)
+    notes = AsyncMock(return_value=[])
+    with (
+        patch("scribe.mcp.tools.search.semantic_search_notes", notes),
+        patch("scribe.mcp.tools.search.semantic_search_systems", AsyncMock(return_value=[])),
+    ):
+        await search(q="x", content_type="system")
+    notes.assert_not_awaited()