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_services_snippets.py

@@ -2,10 +2,18 @@
 from scribe.services import snippets as s
 
 
-def test_compose_title_with_and_without_usage():
-    assert s.compose_title("debounce", "rate-limit a callback") == "debounce — rate-limit a callback"
-    assert s.compose_title("  debounce  ", "") == "debounce"
-    assert s.compose_title("debounce") == "debounce"
+def test_the_embedded_title_joins_the_trigger_the_stored_one_does_not_carry():
+    """Milestone 427: stored title = name; the trigger joins it at embed time,
+    idempotently, so an old composed title comes out the same."""
+    from scribe.services.embeddings import document_title
+
+    data = {"name": "debounce", "when_to_use": "rate-limit a callback"}
+    assert document_title("debounce", "snippet", data) == "debounce — rate-limit a callback"
+    assert document_title("debounce — rate-limit a callback", "snippet", data) == (
+        "debounce — rate-limit a callback"
+    )
+    assert document_title("debounce", "snippet", {"name": "debounce"}) == "debounce"
+    assert document_title("a — note", "note", data) == "a — note"
 
 
 def test_compose_tags_lowercases_language_and_dedups():
@@ -32,7 +40,7 @@ def test_compose_body_bare_code_only():
 
 
 def test_parse_round_trips_a_composed_snippet():
-    title = s.compose_title("useDebouncedRef", "debounce a reactive ref")
+    title = "useDebouncedRef"
     body = s.compose_body(
         code="const x = 1", language="ts", signature="useDebouncedRef(v, ms)",
         when_to_use="debounce a reactive ref", repo="scribe",
@@ -241,9 +249,9 @@ def test_data_and_body_round_trip_to_the_same_fields():
     name, when, sig, lang = ("debounce", "rate-limit a callback",
                              "debounce(fn, ms)", "ts")
     locs = [{"repo": "web", "path": "src/util.ts", "symbol": "debounce"}]
-    # compose_body takes no `name` — the name lives in the title — so the two
-    # serializers get their own argument lists rather than a shared spread.
-    title = s.compose_title(name, when)
+    # compose_body takes no `name` — the name IS the title (milestone 427) — so
+    # the two serializers get their own argument lists rather than a shared spread.
+    title = name
     body = s.compose_body(code="const x = 1", language=lang, signature=sig,
                           when_to_use=when, locations=locs, merged_from=[41, 42])
     tags = s.compose_tags(lang)