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_kind.py

@@ -29,7 +29,6 @@ from types import SimpleNamespace
 
 from scribe.services import knowledge as knowledge_svc
 from scribe.services import lessons as lessons_svc
-from scribe.services import snippets as snippets_svc
 from scribe.services.embeddings import trigger_title
 
 
@@ -76,15 +75,18 @@ def test_one_join_builds_every_trigger_title():
     expected = f"{subject} — {trigger}"
 
     assert trigger_title(subject, trigger) == expected
-    assert lessons_svc.compose_title(subject, trigger) == expected
-    assert snippets_svc.compose_title(subject, trigger) == expected
+    # The two note kinds reach it through the EMBEDDED title (milestone 427),
+    # from their own mirror key — never a hand-rolled join.
+    from scribe.services.embeddings import document_title
+    assert document_title(subject, "lesson", {"when_to_apply": trigger}) == expected
+    assert document_title(subject, "snippet", {"when_to_use": trigger}) == expected
 
 
 def test_a_subject_with_no_trigger_degrades_to_the_subject():
     """It still embeds, just less sharply — an argument for backfilling
     triggers, not for padding the title with whatever text is to hand."""
     assert trigger_title("debounce", "") == "debounce"
-    assert lessons_svc.compose_title("  debounce  ") == "debounce"
+    assert lessons_svc.lesson_document("  debounce  ")[0] == "debounce"
     assert trigger_title("", "when it applies") == "when it applies"