feat(planning): start_planning hands back the active plan that already covers the work (#4079)

Step 4 of milestone 415 "An existing plan is found before a new one is made".
A session that could not see an existing plan made a second one beside it.
start_planning and create_milestone now ask first: an ACTIVE milestone in the
project with the same title, or one that reads as the same plan (title, design
and steps against milestone embeddings), is returned with its progress and a
pointer to create_records(milestone_id=...). Nothing is created; force=true
bypasses.

- dedup.find_matching_plan / plan_gate / plan_match_response; access-checked
  before either arm (rule 78), fail-open like the other gates.
- Done milestones never block; the semantic arm needs 200+ chars of candidate.
- kb_plan_match_threshold (default 0.90) is a setting, in the Settings view,
  and pinned against the Python default by test_settings_defaults_agree.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

tests/test_integration_milestone_search.py

@@ -1,4 +1,4 @@
-"""Real-Postgres tests for finding a plan by meaning (milestone 415, step 3).
+"""Real-Postgres tests for finding a plan by meaning (milestone 415, steps 3 and 4).
 
 A project's roadmap written as milestones was invisible to recall: `search`
 covered notes, tasks and rules, so "is there already a plan for this?" had no
@@ -16,6 +16,7 @@ from scribe.models import async_session
 from scribe.models.embedding import EMBEDDING_DIM, MilestoneEmbedding
 from scribe.models.milestone import Milestone
 from scribe.models.project import Project
+from scribe.services import dedup as dedup_svc
 from scribe.services.embeddings import CHUNKER_VERSION, semantic_search_milestones
 from tests.helpers import ensure_user
 
@@ -80,3 +81,46 @@ async def test_without_a_project_it_searches_the_callers_own(roadmap):
 async def test_a_project_the_caller_cannot_read_returns_nothing(roadmap):
     assert await _found(roadmap["stranger"], project_id=roadmap["mine"]) == []
     assert await _found(roadmap["stranger"]) == []
+
+
+# ── the plan gate (step 4): start_planning finds the plan before making another ──
+
+LONG = "Resolve works and editions against the metadata providers. " * 5
+
+
+async def _gate(roadmap, title, text="", project=None):
+    with patch("scribe.services.embeddings.get_embedding", AsyncMock(return_value=NEAR)):
+        return await dedup_svc.plan_gate(
+            roadmap["owner"], project or roadmap["mine"], title, text,
+        )
+
+
+async def test_the_gate_returns_an_active_plan_with_the_same_title(roadmap):
+    out = await _gate(roadmap, "  m3 — METADATA ")
+    assert out["duplicate"] is True and out["match"] == "title"
+    assert out["existing_milestone"]["id"] == roadmap["m3"]
+    assert f"create_records(milestone_id={roadmap['m3']}" in out["message"]
+
+
+async def test_the_gate_finds_a_plan_by_meaning_and_never_a_done_one(roadmap):
+    """NEAR matches m3 (active), `done` (done) and `foreign` (another project).
+    Only m3 is a plan this project is still working through."""
+    out = await _gate(roadmap, "Book metadata", LONG)
+    assert out["match"] == "semantic"
+    assert out["existing_id"] == roadmap["m3"]
+
+
+async def test_a_done_plan_and_another_projects_plan_do_not_block(roadmap):
+    assert await _gate(roadmap, "Covers") is None
+    assert await _gate(roadmap, "Metadata elsewhere") is None
+    async with async_session() as s:
+        m3 = await s.get(Milestone, roadmap["m3"])
+        m3.status = "done"
+        await s.commit()
+    assert await _gate(roadmap, "Book metadata", LONG) is None
+
+
+async def test_a_short_candidate_is_judged_by_title_alone(roadmap):
+    """A title-only embedding sits in a tight neighbourhood and matches
+    anything nearby, so a bare title never takes the semantic arm."""
+    assert await _gate(roadmap, "Book metadata", "just a title") is None