Lookup index auto-invalidates on KB updates + manual rebuild flag

Diego asked: does the persisted ``archcenter-refs-index.json`` auto-
update when the KB is updated? Partially — mtime on the cache root
catches add/remove of subdirs (POSIX), but NOT changes to files
INSIDE existing subdirs (description.md added, .drawio replaced).

Two fixes:

1. Both KB-update tools now ``touch`` ``kb/diagram/assets/archcenter-refs``
   when they finish a run that produced changes:
   - ``archcenter_description_fetcher.py`` after fetched > 0
   - ``archcenter_zip_downloader.py`` after downloaded / extract /
     overwrite events
   The touch bumps the parent mtime, which the next lookup detects
   and uses to rebuild its persisted index. Closes the silent-drift
   case where description.md fetches landed but the index didn't
   notice.

2. ``archcenter_pattern_lookup.py --rebuild-index`` flag — escape
   hatch for manual KB edits (someone replaces a cached .drawio by
   hand) where neither tool ran. Removes the stale index file before
   the lookup, forcing a cold rebuild.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tools/archcenter_pattern_lookup.py

@@ -402,7 +402,15 @@ def main() -> None:
                         "before scoring (opt-in; requires "
                         "ANTHROPIC_API_KEY). Useful for natural-language "
                         "queries the synonym table doesn't cover.")
+    parser.add_argument("--rebuild-index", action="store_true",
+                        help="Force-rebuild the cached archcenter-refs "
+                        "index (kb/diagram/assets/archcenter-refs-index.json). "
+                        "Use after a manual KB edit if mtime didn't bump.")
     args = parser.parse_args()
+    if args.rebuild_index and CACHE_INDEX_FILE.exists():
+        CACHE_INDEX_FILE.unlink()
+        print(f"[lookup] removed stale index at {CACHE_INDEX_FILE.relative_to(PROJECT_ROOT)}",
+              file=sys.stderr)
 
     queries: list[str] = list(args.queries)
     if args.query: