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_integration_lesson_kind.py

@@ -67,7 +67,7 @@ async def test_a_lesson_is_a_row_the_database_accepts(owner_id):
     value, and stays correct if it is gated with it."""
     lesson = await notes_svc.create_note(
         owner_id,
-        title=lessons_svc.compose_title(SUBJECT, TRIGGER),
+        title=SUBJECT,
         body=f"**When to apply:** {TRIGGER}\n\nOne change at a time.",
         note_type=lessons_svc.LESSON_NOTE_TYPE,
         data={lessons_svc.TRIGGER_KEY: TRIGGER},
@@ -83,7 +83,8 @@ async def test_a_lesson_is_a_row_the_database_accepts(owner_id):
     # and the readable body — because the vector is built from the text and
     # the queries are built from the mirror.
     assert lessons_svc.lesson_trigger(stored) == TRIGGER
-    assert stored.title == f"{SUBJECT} — {TRIGGER}"
+    # The subject alone (milestone 427): the trigger is in `data` and the body.
+    assert stored.title == SUBJECT
     assert "**When to apply:**" in (stored.body or "")
 
 
@@ -97,7 +98,7 @@ async def test_a_lesson_is_not_a_task(owner_id):
     """
     lesson = await notes_svc.create_note(
         owner_id,
-        title=lessons_svc.compose_title(SUBJECT, TRIGGER),
+        title=SUBJECT,
         body="One change at a time.",
         note_type=lessons_svc.LESSON_NOTE_TYPE,
     )