refactor(notes): a snippet's and lesson's stored title is its name; the trigger joins it only in the embedded document (milestone 427)

The title was `subject — trigger` because the stored title WAS the
embedded one, and the join is what makes these kinds rank on the
situation they apply to (#2485). Every surface that shows a title then
showed the trigger too -- menus, lists and search rows ran to kilobytes.

- embeddings.document_title(title, note_type, data, body) joins the
  trigger from `data` (body fallback) at embed time. Idempotent: an
  un-migrated composed title comes out the same, never doubled. The
  embed path, the startup backfill and the dedup gate's semantic signal
  all use it, so the embedded text -- and every vector -- is unchanged.
- Writers store the subject: snippet create/update (service, REST, MCP)
  and lesson_document. Both compose_title helpers are removed.
- Readers: dedup takes `data`; the menus strip the embedded title from a
  passage; list rows project `when_to_use`, which SnippetListView reads.
- 0108 rewrites existing rows on an exact `' — ' || <own trigger>`
  suffix with raw SQL, leaving updated_at alone so the backfill does not
  re-embed the corpus for identical vectors. Downgrade recomposes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/mcp/tools/lessons.py

@@ -168,6 +168,7 @@ async def create_lesson(
         dup = await dedup_svc.find_duplicate_note(
             uid, title, body, project_id=project_id or None,
             is_task=False, note_type=lessons_svc.LESSON_NOTE_TYPE,
+            data=lessons_svc.compose_data(what, when_to_apply),
         )
         if dup is not None:
             return dedup_svc.duplicate_response(dup, "lesson")

src/scribe/mcp/tools/snippets.py

@@ -176,7 +176,9 @@ async def create_snippet(
         raise ValueError("create_snippet requires a non-empty name and code")
     uid = current_user_id()
 
-    title = snippets_svc.compose_title(name, when_to_use)
+    # The NAME is the title (milestone 427); the trigger rides in `data` and
+    # joins the title only in the embedded document.
+    title = name.strip()
     body = snippets_svc.compose_body(
         code=code, language=language, signature=signature,
         when_to_use=when_to_use, repo=repo, path=path, symbol=symbol,
@@ -190,6 +192,7 @@ async def create_snippet(
             # location and code before it compares prose (#2518).
             code=code,
             locations=snippets_svc.resolve_locations(repo, path, symbol, locations),
+            data=snippets_svc.compose_data(name=name, when_to_use=when_to_use),
         )
         if dup is not None:
             return dedup_svc.duplicate_response(dup, "snippet")