Add checkpoint skill, fix temp file collisions, and support starting on existing branches

[sebastientaggart]

Adds the /checkpoint skill for committing and pushing WIP without the full review/merge ceremony, fixes temp file collisions across skills by using unique temp directories per invocation, and updates /start to handle being invoked while already on a feature branch.

Changes across skills:

• checkpoint: New skill — stages, commits with a WIP message, and pushes to remote. Lightweight save point for in-progress work.
• start: Added pre-check for existing feature branches. When invoked on a feature/* branch with new work, prompts the user to continue on the current branch or abort.
• deploy: Updated temp directory pattern to use per-invocation unique directories, preventing collisions when multiple skills run in the same session.
• submit-for-review: Same temp directory collision fix as deploy.

All adapter outputs (Claude, Cursor, Codex, Gemini) regenerated via sync.sh.

Issue #108

[sebastientaggart]

Review Summary

Verdict: APPROVE

Findings

• [NOTE] The /checkpoint skill uses git add -A which stages all files indiscriminately, but the skill text includes an instruction to exclude .env files, secrets, and build artifacts before committing -- this mitigation is adequate since the agent is expected to inspect git status output and handle exclusions.

• [NOTE] Temp directories created under /tmp/CodeCannon/ are never cleaned up after use; over many invocations this will accumulate stale directories, though /tmp is typically purged on reboot so this is low-impact.

No critical or blocking issues found. The three changes (new checkpoint skill, temp directory collision fix, existing-branch pre-check in start) are correct, consistently applied across all four adapter outputs, and match the source templates. The temp directory pattern (mkdir -p /tmp/CodeCannon && mktemp -d /tmp/CodeCannon/XXXXXX) is a sound fix for the collision problem described in the PR.