feat(search): milestones are searchable by meaning — "is there already a plan for this?" (#4078)

`search` covered notes, tasks and rules, and a milestone — the record a plan
lives in — could not be found. A project whose roadmap was written as
milestones had every later plan opened beside the one that already described
it, because nothing could have told the session it existed.

- milestone_embeddings (migration 0102): the third sibling of note_ and
  rule_embeddings, for note 3163's reason — the search is milestone-specific.
  The document is title — description, then description and the plan body,
  so a roadmap milestone with no description is still found by its design.
- Written on create, on a title/description/body update, and for a plan made
  through start_planning / create_records, fire-and-forget with the parent-row
  claim (#3262); a startup backfill covers every existing milestone. Derived,
  so it joins _NOT_INCLUDED beside the other embeddings.
- semantic_search_milestones: a project's milestones when the caller can read
  it (access.can_read_project), otherwise the caller's own; optional status.
- search(content_type="milestone"): id, title, description, status, project
  and progress. Its own shape, and not part of "all", whose results are
  note-shaped. The docstring says what it is for: ask before start_planning.
- Integration test on real Postgres: found in its project and not another,
  status narrows, an unreadable project returns nothing.

Milestone 415 step 3.

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

tests/test_mcp_tool_search.py

@@ -106,3 +106,34 @@ async def test_rule_search_scopes_to_the_project_it_is_given(project_id, scope):
     kwargs = found.await_args.kwargs
     assert {k: kwargs[k] for k in scope} == scope
     assert set(kwargs) & {"project_id", "everywhere"} == set(scope)
+
+
+def test_a_milestone_is_embedded_by_what_it_is_for_then_its_plan():
+    from scribe.services.embeddings import milestone_document
+
+    assert milestone_document("M3", "metadata providers", "## Goal\nx") == (
+        "M3 — metadata providers", "metadata providers\n\n## Goal\nx")
+    # A roadmap milestone written with no description is still findable by its plan.
+    assert milestone_document("M3", None, "the plan") == ("M3", "the plan")
+    assert milestone_document(None, None, None) == (None, None)
+
+
+@pytest.mark.asyncio
+async def test_milestone_search_is_its_own_shape_and_scopes_to_the_project():
+    """milestone 415: 'is there already a plan for this?' has a tool."""
+    from unittest.mock import MagicMock
+
+    _user_id_ctx.set(7)
+    ms = MagicMock(id=339, title="M3 — Metadata", description="works, editions",
+                   status="active", project_id=30)
+    found = AsyncMock(return_value=[(0.81, ms)])
+    summary = AsyncMock(return_value=[{"id": 339, "total": 0, "completed": 0}])
+    with patch("scribe.mcp.tools.search.semantic_search_milestones", found), \
+         patch("scribe.services.milestones.get_project_milestone_summary", summary):
+        out = await search(q="book metadata", content_type="milestone", project_id=30)
+    assert found.await_args.kwargs["project_id"] == 30
+    assert out["results"] == [{
+        "id": 339, "title": "M3 — Metadata", "description": "works, editions",
+        "status": "active", "project_id": 30, "total": 0, "completed": 0,
+        "similarity": 0.81,
+    }]