fix(evidence): reconcile generations safely

harness/tht/adapters/vector/pgvector.py

@@ -87,6 +87,7 @@ class PgVectorStore:
             upsert=writable,
             metadata_filter=self._reader is not None,
             delete_generation=writable,
+            list_evidence_generations=writable,
         )
 
     def _probe(
@@ -428,5 +429,26 @@ class PgVectorStore:
             if raw is not None:
                 raw.close()
 
+    def list_evidence_generations(self, collection: str) -> list[str]:
+        if collection != "evidence":
+            raise VectorStoreError("Only exact Evidence generations may be listed")
+        raw = None
+        try:
+            raw = self._require_writer().raw_connection()
+            with raw.cursor() as cursor:
+                cursor.execute(
+                    sql.SQL(
+                        "SELECT DISTINCT metadata->>'vector_generation' FROM {} "
+                        "WHERE kind = 'evidence' AND metadata->>'vector_generation' "
+                        "~ '^gen:[0-9a-f]{{32}}$' ORDER BY 1"
+                    ).format(_collection(self._schema, collection))
+                )
+                return [row[0] for row in cursor.fetchall()]
+        except Exception as exc:
+            raise VectorWriteUnavailable("Vector generation inventory unavailable") from exc
+        finally:
+            if raw is not None:
+                raw.close()
+
 
 __all__ = ["ALLOWED_COLLECTIONS", "PgVectorStore"]