Add Git conventions section to CLAUDE.md

Document use of "Closes #XX" in commit messages or PR descriptions
when working on branches that reference GitHub issues.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: patcon

CLAUDE.md

@@ -62,3 +62,7 @@ Tests use real Polis API data fixtures in `tests/fixtures/`. The test suite incl
 - `plots`: matplotlib, seaborn, concave-hull (visualization)
 - `dev`: pytest, mkdocs, nbmake (development)
 - `all`: everything
+
+## Git Conventions
+
+- When working on a branch that references an issue (e.g., `116-fix-bestkmeans`), include `Closes #116` in the commit message or PR description to auto-close the issue when merged