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

src/scribe/mcp/tools/search.py

@@ -12,7 +12,8 @@ import time
 from scribe.mcp._context import current_user_id
 from scribe.services.access import owner_names_for
 from scribe.services.embeddings import (
-    DEFAULT_SIMILARITY_THRESHOLD, semantic_search_notes, semantic_search_rules,
+    DEFAULT_SIMILARITY_THRESHOLD, semantic_search_milestones, semantic_search_notes,
+    semantic_search_rules,
 )
 from scribe.services import rulebooks as rulebooks_svc
 from scribe.services.retrieval_telemetry import record_retrieval, retrieval_summary
@@ -67,6 +68,40 @@ async def _search_rules(uid: int, q: str, limit: int, project_id: int) -> dict:
     }
 
 
+async def _search_milestones(uid: int, q: str, limit: int, project_id: int) -> dict:
+    """Milestones by meaning — "is there already a plan for this?" (milestone 415).
+
+    Its own result shape, like rules: a milestone is a plan with progress, not
+    a note with a body. The plan itself is left out — get_milestone reads it —
+    because a search hit is for recognising a plan, and bodies run long.
+    Not part of content_type="all", whose results are note-shaped.
+    """
+    raw = await semantic_search_milestones(uid, q, project_id=project_id or None, limit=limit)
+    progress: dict[int, dict] = {}
+    if raw:
+        from scribe.services import milestones as milestones_svc
+
+        for pid in {m.project_id for _s, m in raw}:
+            for row in await milestones_svc.get_project_milestone_summary(uid, pid):
+                progress[row["id"]] = row
+    return {
+        "results": [
+            {
+                "id": m.id,
+                "title": m.title,
+                "description": m.description or "",
+                "status": m.status,
+                "project_id": m.project_id,
+                "total": progress.get(m.id, {}).get("total", 0),
+                "completed": progress.get(m.id, {}).get("completed", 0),
+                "similarity": float(score),
+            }
+            for score, m in raw
+        ],
+        "total": len(raw),
+    }
+
+
 async def search(
     q: str,
     content_type: str = "all",
@@ -93,7 +128,12 @@ async def search(
             tagging?". A hit carries the rule's `why` and `how_to_apply`,
             which the session-start payload does not. With a project_id,
             rules come back as the global rules plus that project's own;
-            with 0, every rule in the rulebook.
+            with 0, every rule in the rulebook. Or 'milestone' (PLANS):
+            reach for it before start_planning to ask whether a plan for
+            this work already exists — a match is where new steps go
+            (create_records(milestone_id=…)), not a reason to open a second
+            milestone. Hits carry title, description, status and progress;
+            get_milestone reads the plan. Not included in 'all'.
         limit: maximum number of results (1-50).
         project_id: Scope results to one project. PASS THE ACTIVE PROJECT'S ID
             whenever a project is in scope (the one you entered with
@@ -118,6 +158,8 @@ async def search(
     limit = max(1, min(limit, 50))
     if content_type == "rule":
         return await _search_rules(uid, q, limit, project_id)
+    if content_type == "milestone":
+        return await _search_milestones(uid, q, limit, project_id)
     is_task = {"note": False, "task": True}.get(content_type)  # None => any
     t0 = time.perf_counter()
     report: dict = {}