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>

kb/diagram/assets/archcenter-refs-index.json

@@ -1,5 +1,5 @@
 {
-  "cache_dir_mtime": 1777161774.8360987,
+  "cache_dir_mtime": 1777171058.9167857,
   "entries": {
     "active-data-guard-far-sync-dba-oracle": {
       "drawio": "kb/diagram/assets/archcenter-refs/active-data-guard-far-sync-dba-oracle/active-data-guard-far-sync-dba.drawio",