refactor(embeddings): one definition of the document a record is embedded as

`f"{title}\n{body}"` was written out four times. #2486 found three — the write
path, the recurring-task spawn, the startup backfill. The guard added here
found the fourth immediately, and it was the one that mattered most.

`dedup.find_duplicate_note` built the same string as a QUERY, compared against
embedded documents. Shaped differently from the corpus it searches, the gate
degrades silently: it still returns neighbours, just less apt ones, and nothing
says the query and the index stopped agreeing. The spawn path has the same
shape of risk — a recurring task embedded differently from everything else is
ranked against documents it doesn't match.

None of the four had diverged. That is what makes this worth doing now rather
than after: they are identical today, so collapsing them is a no-op, and the
whole point is that the next change to the shape can't hit three of four.

Which is imminent. #2486 measured a dev-log separating from five unrelated
dev-logs by 0.023 where a snippet separates by 0.153 — the difference being
that a snippet states its purpose twice in a short document. Whether that shape
is right is the open question; testing an alternative against four copies would
mean testing a shape that isn't the one in production. This is the precondition
the issue named.

The guard is source inspection, matching the f-string pattern rather than a
variable name, so a copy that renames its locals is still caught. A behavioural
test cannot see this: an inlined copy produces the same string today and
diverges the day the shape changes.

Refs #2486

src/scribe/services/dedup.py

@@ -258,7 +258,13 @@ async def find_duplicate_note(
 
     # --- Signal 3: semantic similarity (only with a substantial body) ---
     if body and len(body.strip()) >= _MIN_BODY_FOR_SEMANTIC:
-        query = f"{title}\n{body}".strip()
+        # Built by the SAME function the corpus was embedded with. This one is
+        # the copy that mattered most and was easiest to miss: it is a QUERY
+        # document, compared against embedded ones. Shaped differently from the
+        # corpus it searches, the gate degrades silently — it still returns
+        # neighbours, just less apt ones, and no signal says the query and the
+        # index stopped agreeing (found by the guard in test_embedding_text).
+        query = embeddings_svc.embedding_text(title, body)
         # Scope the semantic check the same way as the title check: a record in
         # project P compares only to P; a project-less (orphan) record compares
         # only to other orphans (orphan_only), NOT across every project — without