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/services/snippets.py

@@ -54,18 +54,6 @@ UNSET: object = object()
 
 # --- serialize: structured fields -> note (title/body/tags) ------------------
 
-def compose_title(name: str, when_to_use: str = "") -> str:
-    """`name — when to use` (or just `name` when no usage note is given).
-
-    The join itself lives in `embeddings.trigger_title`, which rules and
-    lessons build their titles from too. Kept as a named function here because
-    it is this module's public vocabulary and callers say `compose_title`.
-    """
-    from scribe.services.embeddings import trigger_title
-
-    return trigger_title(name, when_to_use)
-
-
 def compose_tags(language: str = "", tags: list[str] | None = None) -> list[str]:
     """Language (lowercased) first, then the `snippet` marker, then caller tags —
     de-duplicated, order preserved."""
@@ -741,7 +729,7 @@ async def create_snippet(
     locations = resolve_locations(repo, path, symbol, locations)
     note = await notes_svc.create_note(
         user_id,
-        title=compose_title(name, when_to_use),
+        title=name.strip(),
         body=compose_body(
             code=code, language=language, signature=signature,
             when_to_use=when_to_use, locations=locations,
@@ -898,7 +886,7 @@ async def update_snippet(
         provenance = cur.get("provenance")
 
     fields: dict = {
-        "title": compose_title(merged["name"], merged["when_to_use"]),
+        "title": (merged["name"] or "").strip(),
         "body": compose_body(
             code=merged["code"], language=merged["language"],
             signature=merged["signature"], when_to_use=merged["when_to_use"],