chore: add CLAUDE.md, stop tracking egg-info build artifacts

- Add CLAUDE.md (Claude Code orientation for the repo).
- Remove app/src/ctxd.egg-info/* from version control and gitignore
  *.egg-info/ — it is regenerated by `pip install -e` and only dirties
  the working tree.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -20,3 +20,6 @@ app/.env
 # Python cache
 __pycache__/
 *.py[cod]
+
+# Build artifacts (regenerated by `pip install -e`)
+*.egg-info/