Add branch protection and multi-agent coordination guard

[CodeMonkeyCybersecurity]

Problem

The divergent branches incident (192 local vs 5 remote commits on main) was caused by parallel development without coordination. Two agents/sessions pushed different work to main simultaneously, creating merge conflicts in critical files.

Impact

• Development blocked until manual conflict resolution
• Risk of lost work if resolved incorrectly
• No automated detection of divergence before it becomes severe

Proposed Solution

Short-term (local)

1. Configure pull.rebase = true repo-wide (done in fe4da19)
2. Add pre-push hook that warns when >10 commits ahead of remote
3. Add pre-push hook that fetches and warns on divergence before push

Medium-term (CI)

4. Add branch protection rules requiring PR reviews
5. Enforce linear history (no merge commits) via branch protection
6. Add GitHub Actions check that validates branch is up-to-date before merge

Long-term (coordination)

7. Implement COORDINATION.md lockfile pattern for multi-agent work
8. Each agent claims a branch scope before starting work
9. Automated alerts when two agents touch the same files

Evidence

• Git log shows 192 local commits diverged from 5 remote commits
• Common ancestor: 93df31dd
• VSCode error: fatal: Need to specify how to reconcile divergent branches
• 4 files conflicted: .gitignore, package.json, cmd/backup/chats.go, ci.yml

Acceptance Criteria

• Pre-push hook warns on divergence
• Branch protection enabled on main
• Linear history enforced