fix(bigmind): fix FTS sync in delete_chunks_before — use rowid DELETE not rebuild

conversation_chunks_fts is a standalone FTS5 table (no content= option).
The old INSERT ... VALUES('rebuild') is a no-op on standalone tables and
left deleted chunks searchable in the FTS shadow tables.

Fix: collect IDs before deletion, explicitly DELETE FROM conversation_chunks_fts
WHERE rowid IN (...) before removing from the main table. This keeps FTS
in sync after every vacuum call.

Tests: 303/303 passing. Vacuum tests now pass for the right reason.

mcp/bigmind/bigmind/memory_store.py

@@ -441,21 +441,28 @@ def search_chunks(user_id: str, query: str, limit: int = 10) -> list:
 def delete_chunks_before(user_id: str, cutoff_iso: str) -> int:
     """Delete Tier-3 chunks older than cutoff. Returns count deleted."""
     with db() as conn:
-        count = conn.execute(
-            "SELECT COUNT(*) FROM conversation_chunks WHERE user_id=? AND created_at < ?",
+        # Collect IDs first — needed for FTS sync.
+        # conversation_chunks_fts is a STANDALONE FTS5 table (no content= option),
+        # so we must delete FTS rows explicitly by rowid. The old VALUES('rebuild')
+        # approach only works for content= backed tables and was a no-op here.
+        rows = conn.execute(
+            "SELECT id FROM conversation_chunks WHERE user_id=? AND created_at < ?",
             (user_id, cutoff_iso),
-        ).fetchone()[0]
-        if count == 0:
+        ).fetchall()
+        if not rows:
             return 0
+        ids = [r[0] for r in rows]
+        # Delete FTS entries by rowid before removing from main table
+        placeholders = ",".join("?" * len(ids))
+        conn.execute(
+            f"DELETE FROM conversation_chunks_fts WHERE rowid IN ({placeholders})",
+            ids,
+        )
         conn.execute(
             "DELETE FROM conversation_chunks WHERE user_id=? AND created_at < ?",
             (user_id, cutoff_iso),
         )
-        # Rebuild the FTS5 index from the content table — always correct for content= tables
-        conn.execute(
-            "INSERT INTO conversation_chunks_fts(conversation_chunks_fts) VALUES('rebuild')"
-        )
-        return count
+        return len(ids)
 
 
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