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_derived_mirror_generic_door.py

@@ -164,11 +164,11 @@ def _lesson_body(trigger, insight="Read the job log.", sources=None):
 
 @pytest.mark.asyncio
 async def test_a_body_write_moves_a_lessons_trigger_with_it():
-    from scribe.services.lessons import TRIGGER_KEY, compose_title
+    from scribe.services.lessons import TRIGGER_KEY
 
     what = "Read the job log before waiting longer"
     note = fake_lesson(
-        title=compose_title(what, NEW_TRIGGER),
+        title=what,
         data={TRIGGER_KEY: "a CI run is slow", "what": what},
         project_id=None,
     )
@@ -183,13 +183,16 @@ async def test_a_body_write_moves_a_lessons_trigger_with_it():
 @pytest.mark.asyncio
 async def test_a_subject_containing_an_em_dash_still_splits():
     """Why `untrigger_title` is given the trigger instead of splitting on the
-    separator: a subject may legitimately contain one."""
-    from scribe.services.lessons import TRIGGER_KEY, compose_title
+    separator: a subject may legitimately contain one. The title here is an
+    UN-MIGRATED one, still carrying its trigger (milestone 427), because that
+    is the row the inverse still has to read correctly."""
+    from scribe.services.embeddings import trigger_title
+    from scribe.services.lessons import TRIGGER_KEY
 
     what = "A wait with no deadline — the shape, not the symptom"
     trigger = "you are about to await something crossing a process boundary"
     note = fake_lesson(
-        title=compose_title(what, trigger), data=None, project_id=None,
+        title=trigger_title(what, trigger), data=None, project_id=None,
     )
     await _update(note, body=_lesson_body(trigger))
     assert note.data["what"] == what
@@ -202,10 +205,10 @@ async def test_dropping_the_provenance_line_drops_it_from_the_mirror():
     the failure this recompose exists to prevent, not a courtesy — the
     opposite call from a snippet's `verification`, which is carried because it
     was never in the body to delete."""
-    from scribe.services.lessons import SOURCES_KEY, compose_title
+    from scribe.services.lessons import SOURCES_KEY
 
     note = fake_lesson(
-        title=compose_title("Something learned", "a situation"),
+        title="Something learned",
         data={SOURCES_KEY: [999]},
         project_id=None,
     )
@@ -229,7 +232,7 @@ async def test_an_explicit_data_wins_for_a_lesson_too():
 async def test_a_lesson_title_change_reaches_the_mirror():
     """A lesson's subject lives in its title, so a title edit is a trigger for
     recomposition exactly as it is for a snippet's name."""
-    from scribe.services.lessons import TRIGGER_KEY, compose_title
+    from scribe.services.lessons import TRIGGER_KEY
 
     trigger = "two absolute siblings overlap"
     note = fake_lesson(
@@ -237,7 +240,7 @@ async def test_a_lesson_title_change_reaches_the_mirror():
         data={TRIGGER_KEY: trigger, "what": "the old subject"},
         project_id=None,
     )
-    await _update(note, title=compose_title("the new subject", trigger))
+    await _update(note, title="the new subject")
     assert note.data["what"] == "the new subject"
     assert note.data[TRIGGER_KEY] == trigger