fix: remove stale worktree submodule + ignore .claude/ dir

The agent worktree left a ghost submodule entry that was causing
GitHub Pages CI to fail with "No url found for submodule path".
Also strips HF frontmatter from GitHub README (push_to_hf.sh adds
it automatically when deploying to HF Space).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/worktrees/agent-a1f6aaf9

@@ -1 +0,0 @@
-Subproject commit cd035f4c8bdc4d423d1f66ea8d62d059fa76ddfc

.gitignore

@@ -34,3 +34,4 @@ Thumbs.db
 *.log
 tmp/
 temp/
+.claude/