chore: post-Phase-1 cleanup — gitignore, packaging, backlog

- .gitignore: add *.egg-info/ so editable installs don't show in git status.
- pyproject.toml: add [build-system] and [tool.setuptools.packages.find]
  scoped to chat*, fixing pip install -e . which was failing on data/
  auto-discovery.
- CLAUDE.md: add Phase 1.5 cleanup backlog section under Phase 1 status,
  capturing the small follow-ups surfaced in implementer reviews
  (open_db refactor, regenerate SSE broadcast, you-activity purge,
  drawer edits for deferred fields, NICE trim order).

.gitignore

@@ -8,3 +8,4 @@ data/
 __pycache__/
 *.pyc
 .pytest_cache/
+*.egg-info/