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_mcp_tool_planning.py

@@ -7,6 +7,13 @@ from tests.helpers import fake_task
 pytestmark = pytest.mark.usefixtures("_bind_user")
 
 
+@pytest.fixture(autouse=True)
+def _no_plan_gate():
+    """The plan gate reads the database; the tests that are about it re-patch it."""
+    with patch("scribe.mcp.tools.tasks.dedup_svc.plan_gate", AsyncMock(return_value=None)):
+        yield
+
+
 @pytest.mark.asyncio
 async def test_start_planning_tool_delegates_to_service():
     payload = {"milestone": {"id": 5}, "applicable_rules": [], "project_rules": [],
@@ -23,6 +30,42 @@ async def test_start_planning_tool_delegates_to_service():
     }
 
 
+@pytest.mark.asyncio
+async def test_start_planning_returns_the_plan_that_already_covers_it():
+    """The FabledLibrarian failure (milestone 415): a session that could not see
+    the existing plan made a second one. Now it is handed the first, and
+    nothing is created."""
+    match = {"duplicate": True, "existing_id": 12}
+    svc = AsyncMock()
+    with patch("scribe.mcp.tools.tasks.dedup_svc.plan_gate",
+               AsyncMock(return_value=match)) as gate, \
+         patch("scribe.mcp.tools.tasks.dedup_svc.find_duplicate_note", AsyncMock()) as steps, \
+         patch("scribe.mcp.tools.tasks.planning_svc.start_planning", svc):
+        from scribe.mcp.tools.tasks import start_planning
+        out = await start_planning(project_id=3, title="Metadata", body="design",
+                                   steps=[{"title": "Resolve editions", "body": "via providers"}])
+    assert out is match
+    svc.assert_not_awaited()
+    steps.assert_not_awaited()
+    # The candidate is judged by its steps as well as its design.
+    assert gate.call_args.args[:3] == (7, 3, "Metadata")
+    assert "design" in gate.call_args.args[3]
+    assert "Resolve editions\nvia providers" in gate.call_args.args[3]
+
+
+@pytest.mark.asyncio
+async def test_force_creates_the_plan_without_asking_the_gate():
+    gate = AsyncMock(return_value={"duplicate": True})
+    with patch("scribe.mcp.tools.tasks.dedup_svc.plan_gate", gate), \
+         patch("scribe.mcp.tools.tasks.planning_svc.start_planning",
+               AsyncMock(return_value={"milestone": {"id": 5}})) as svc:
+        from scribe.mcp.tools.tasks import start_planning
+        out = await start_planning(project_id=3, title="Metadata", force=True)
+    assert out["milestone"]["id"] == 5
+    svc.assert_awaited_once()
+    gate.assert_not_awaited()
+
+
 @pytest.mark.asyncio
 async def test_get_task_augments_plan_with_rules():
     applicable = {"rules": [{"id": 1, "title": "r"}], "truncated": False,