Research crews can create PRs despite post-process safety guard

[khang859]

Research crews are creating pull requests when they shouldn't. The Hull safety guard runs post-process and cannot undo what agents have already pushed to origin.

Root Cause: Agents have unrestricted Bash access and can call git push + gh pr create directly during execution, before Hull cleanup runs.

Recommended Fixes:

1. Add research-crew warning to worktreeWarning in hull.ts (explicit no-push/PR instruction)
2. Fix Fleet skill language to clarify PRs are code-missions-only
3. Write research-blocking settings.json before spawn
4. Add Sentinel anomaly alert for research+pr_branch
5. Fix git reset to use --hard

See docs/learnings/research-crew-pr-creation-issue.md for full investigation.