Enforce meaningful branch names via a PreToolUse…

Details

Work kept landing on auto-generated `claude/*` placeholder branches because
the only safeguard was prose in CLAUDE.md, which loses to the web/Cloud
harness's explicit "develop on the designated `claude/...` branch"
instruction. Make the rule deterministic instead of advisory.
- Add `.claude/hooks/guard-branch-names.py`: a PreToolUse hook that denies
  `git push` and GitHub `create_pull_request` when the branch is a `claude/*`
  placeholder, with a message telling Claude to rename to a meaningful
  `type/short-description` branch first. Detection is segment-aware (catches
  loop-wrapped / chained / env-prefixed pushes, bare/HEAD pushes via the
  current branch, and the MCP PR head) while ignoring mere mentions inside
  echo/grep/comments. Branch deletions (`git push --delete`, `:branch`) stay
  allowed for cleanup.
- Wire it into `.claude/settings.json` (matcher `Bash|mcp__github__create_pull_request`),
  preserving the existing Stop hook.
- Strengthen the CLAUDE.md git-conventions section to explicitly resolve the
  conflict: the harness's designated-branch instruction governs where commits
  land, not the name — rename before the first push/PR.
Verified: 24-case unit battery on the hook (placeholder push/PR denied,
meaningful allowed, deletions allowed, echo/grep mentions not blocked); a live
in-session push of a `claude/*` branch is blocked while a `feat/*` push passes;
and 4 sandbox agents started on placeholder branches all shipped from
meaningful names (fix/…, docs/…), renaming proactively.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Siw4HBDfxn2sBiDFBpNkPN

Files changed (3)