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_services_dedup.py

@@ -4,10 +4,15 @@ from unittest.mock import AsyncMock, MagicMock, patch
 import pytest
 
 from scribe.services.dedup import (
+    PLAN_MATCH_DEFAULT_THRESHOLD,
     DuplicateMatch,
     duplicate_response,
     find_duplicate_note,
     find_duplicate_rule,
+    find_matching_plan,
+    get_plan_match_threshold,
+    plan_candidate_text,
+    plan_match_response,
 )
 from tests.helpers import fake_note, make_mock_session
 
@@ -342,3 +347,78 @@ def test_every_kind_has_a_suggestion_and_none_proposes_merging_notes():
     assert "merge" in _KIND_SUGGESTION["snippet"]
     assert "NOT merge" in _KIND_SUGGESTION["note"]
     assert "supersedes" in _KIND_SUGGESTION["note"]
+
+
+# ── the plan gate (milestone 415, step 4) ─────────────────────────────────────
+
+
+@pytest.mark.asyncio
+async def test_plan_title_match_short_circuits_the_semantic_arm():
+    ms = MagicMock(id=415, title="Plan gate")
+    sem = AsyncMock()
+    with patch("scribe.services.dedup.can_read_project", AsyncMock(return_value=True)), \
+         patch("scribe.services.dedup.async_session", return_value=_session_returning(ms)), \
+         patch("scribe.services.dedup.embeddings_svc.semantic_search_milestones", sem):
+        dup = await find_matching_plan(7, 2, " plan  GATE", "x" * 300)
+    assert (dup.id, dup.reason, dup.similarity) == (415, "title", 1.0)
+    sem.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_plan_semantic_arm_asks_for_active_plans_in_the_project_at_the_setting():
+    ms = MagicMock(id=9, title="Metadata")
+    sem = AsyncMock(return_value=[(0.912345, ms)])
+    with patch("scribe.services.dedup.can_read_project", AsyncMock(return_value=True)), \
+         patch("scribe.services.dedup.async_session", return_value=_session_returning(None)), \
+         patch("scribe.services.dedup.embeddings_svc.semantic_search_milestones", sem), \
+         patch("scribe.services.settings.get_setting", AsyncMock(return_value="0.8")):
+        dup = await find_matching_plan(7, 2, "Book metadata", "x" * 300)
+    assert (dup.id, dup.reason, dup.similarity) == (9, "semantic", 0.912)
+    kw = sem.call_args.kwargs
+    assert (kw["project_id"], kw["status"], kw["threshold"]) == (2, "active", 0.8)
+
+
+@pytest.mark.asyncio
+async def test_plan_gate_fails_open():
+    boom = MagicMock(side_effect=RuntimeError("db down"))
+    with patch("scribe.services.dedup.can_read_project", AsyncMock(return_value=True)), \
+         patch("scribe.services.dedup.async_session", boom):
+        assert await find_matching_plan(7, 2, "Anything", "x" * 300) is None
+    with patch("scribe.services.dedup.can_read_project", AsyncMock(side_effect=RuntimeError)):
+        assert await find_matching_plan(7, 2, "Anything", "x" * 300) is None
+    assert await find_matching_plan(7, 0, "No project") is None
+
+
+@pytest.mark.asyncio
+async def test_plan_gate_says_nothing_about_a_project_the_caller_cannot_read():
+    ms = MagicMock(id=415, title="Their plan")
+    session = MagicMock(return_value=_session_returning(ms))
+    with patch("scribe.services.dedup.can_read_project", AsyncMock(return_value=False)), \
+         patch("scribe.services.dedup.async_session", session):
+        assert await find_matching_plan(8, 2, "Their plan") is None
+    session.assert_not_called()
+
+
+@pytest.mark.asyncio
+async def test_a_bad_threshold_setting_falls_back_to_the_default():
+    with patch("scribe.services.settings.get_setting", AsyncMock(return_value="lots")):
+        assert await get_plan_match_threshold(7) == PLAN_MATCH_DEFAULT_THRESHOLD
+    with patch("scribe.services.settings.get_setting", AsyncMock(return_value="7")):
+        assert await get_plan_match_threshold(7) == 1.0
+
+
+def test_plan_candidate_text_carries_the_steps():
+    text = plan_candidate_text(description=None, body="  design ", step_texts=["Step one\n", ""])
+    assert text == "design\n\nStep one"
+
+
+def test_plan_match_response_points_at_adding_steps_not_a_second_plan():
+    out = plan_match_response(DuplicateMatch(12, "Metadata", 0.93, "semantic"),
+                              {"total": 5, "completed": 2, "description": "providers"})
+    assert out["duplicate"] is True and out["existing_id"] == 12
+    assert out["existing_milestone"] == {
+        "id": 12, "title": "Metadata", "description": "providers", "total": 5, "completed": 2,
+    }
+    for phrase in ("create_records(milestone_id=12", "get_milestone(12)", "force=true",
+                   "2 of 5 steps"):
+        assert phrase in out["message"]