Fix find_files slowness with SQLite FindIndex (#354).

[false200]

Fixes #354

So basically I added a new file, app/utils/file_index.py, that builds a SQLite index of filenames (FTS5 trigram on the basename). It lives at {base_directory}/.craftbot/findindex.db.

From the outside, find_files behaves the same:

• empty base_directory still goes to home
• the agent can still search wherever it wants
• we still crawl everything, including stuff like $Recycle.Bin
• the response is still just status, matches, and message - all matches, no head_limit

The difference is recursive searches hit the index instead of running os.walk every single time. Non recursive is unchanged - still just a top level glob.

After the first build, watchdog keeps the index updated with a 5 second debounce.

Also added **/.craftbot/ to .gitignore so those runtime DB files don't get committed.

[false200]

@makiroll1125 I've pushed an improvement for this issue. There isn't really a single best solution since each approach has tradeoffs, but this should make search work better than the previous one. Would appreciate a review.