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/dedup.py

@@ -248,6 +248,7 @@ async def find_duplicate_note(
     note_type: str = "note",
     code: str = "",
     locations: list[dict] | None = None,
+    data: dict | None = None,
 ) -> DuplicateMatch | None:
     """Best near-duplicate of (title, body) within the same owner + project +
     kind, or None. Title match first (cheap, exact), then — for snippets — the
@@ -258,6 +259,11 @@ async def find_duplicate_note(
     `code` and `locations` are the snippet's structured fields. They are ignored
     for every other kind, and passing them is what lets the gate compare
     ARTEFACTS rather than descriptions of artefacts (#2518).
+
+    `data` is the candidate's structured mirror. For a snippet or lesson it
+    carries the trigger, which the TITLE no longer does (milestone 427): the
+    title check compares names, and the semantic check rebuilds the embedded
+    document from `data`.
     """
     norm = " ".join((title or "").split()).lower()
 
@@ -309,7 +315,11 @@ async def find_duplicate_note(
         # section. Capped so one pathological paste can't turn a save into
         # dozens of searches — a duplicate past the cap is the duplicate
         # report's job, not the gate's.
-        for query in embeddings_svc.chunk_document(title, body)[:_GATE_MAX_CHUNKS]:
+        # The EMBEDDED title (milestone 427): a snippet or lesson is stored
+        # under its name and embedded under `name — trigger`, so the query
+        # document is built the way the corpus was, from `data`.
+        doc_title = embeddings_svc.document_title(title, note_type, data, body)
+        for query in embeddings_svc.chunk_document(doc_title, body)[:_GATE_MAX_CHUNKS]:
             # Scope the semantic check the same way as the title check: a record
             # in project P compares only to P; a project-less (orphan) record
             # compares only to other orphans (orphan_only), NOT across every