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>

tests/test_lesson_rest_door.py

@@ -214,13 +214,13 @@ def test_the_payload_reads_back_the_composed_fields():
     """A caller that wrote `when_to_apply` reads `when_to_apply` back, not a
     body it has to parse."""
     from tests.helpers import fake_lesson
-    from scribe.services.lessons import compose_body, compose_title, lesson_to_dict
+    from scribe.services.lessons import compose_body, lesson_to_dict
 
     what = "Read the job log before waiting longer"
     trigger = "a CI run has sat in_progress longer than its suite takes"
     note = fake_lesson(
         id=7,
-        title=compose_title(what, trigger),
+        title=what,
         body=compose_body("The work is usually done.", trigger, [4181]),
         data={"what": what, "when_to_apply": trigger, "taught_by": [4181]},
         project_id=None,