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_milestones.py

@@ -12,6 +12,15 @@ from tests.helpers import fake_milestone
 pytestmark = pytest.mark.usefixtures("_bind_user")
 
 
+@pytest.fixture(autouse=True)
+def _no_plan_gate():
+    """The plan gate reads the database; these tests are about what reaches
+    the service. The gate's own tests re-patch it."""
+    with patch("scribe.mcp.tools.milestones.dedup_svc.plan_gate",
+               AsyncMock(return_value=None)):
+        yield
+
+
 @pytest.mark.asyncio
 async def test_list_milestones_returns_dict_with_progress():
     rows = [{"id": 1, "title": "MS1", "status": "active", "total": 2}]
@@ -63,6 +72,33 @@ async def test_create_milestone_empty_body_becomes_none():
     assert mock.call_args.kwargs["body"] is None
 
 
+@pytest.mark.asyncio
+async def test_create_milestone_returns_the_active_plan_that_already_covers_it():
+    match = {"duplicate": True, "existing_id": 4}
+    create = AsyncMock()
+    with patch("scribe.mcp.tools.milestones.dedup_svc.plan_gate",
+               AsyncMock(return_value=match)) as gate, \
+         patch("scribe.mcp.tools.milestones.milestones_svc.create_milestone", create):
+        out = await create_milestone(project_id=1, title="t", description="d", body="b")
+    assert out is match
+    create.assert_not_awaited()
+    assert gate.call_args.args[:3] == (7, 1, "t")
+    assert gate.call_args.args[3] == "d\n\nb"
+
+
+@pytest.mark.asyncio
+async def test_create_milestone_skips_the_gate_when_forced_or_done():
+    """force: the caller read the match. done: a record of past work is not a
+    plan competing with an open one."""
+    gate = AsyncMock(return_value={"duplicate": True})
+    create = AsyncMock(return_value=fake_milestone(id=6))
+    with patch("scribe.mcp.tools.milestones.dedup_svc.plan_gate", gate), \
+         patch("scribe.mcp.tools.milestones.milestones_svc.create_milestone", create):
+        assert (await create_milestone(project_id=1, title="t", force=True))["id"] == 6
+        assert (await create_milestone(project_id=1, title="t", status="done"))["id"] == 6
+    gate.assert_not_awaited()
+
+
 @pytest.mark.asyncio
 async def test_update_milestone_sends_body():
     m = fake_milestone()