chore(ci): establish PR governance for agent collaboration (#177)

* chore(ci): establish PR governance for agent collaboration

* docs: add AGENTS playbook and strengthen agent collaboration workflow

---------

Co-authored-by: chumyin <183474434+chumyin@users.noreply.github.com>

.github/pull_request_template.md

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+## Summary
+
+Describe this PR in 2-5 bullets:
+
+- Problem:
+- Why it matters:
+- What changed:
+- What did **not** change (scope boundary):
+
+## Change Type
+
+- [ ] Bug fix
+- [ ] Feature
+- [ ] Refactor
+- [ ] Docs
+- [ ] Security hardening
+- [ ] Chore / infra
+
+## Scope
+
+- [ ] Core runtime / daemon
+- [ ] Provider integration
+- [ ] Channel integration
+- [ ] Memory / storage
+- [ ] Security / sandbox
+- [ ] CI / release / tooling
+- [ ] Documentation
+
+## Linked Issue
+
+- Closes #
+- Related #
+
+## Testing
+
+Commands and result summary (required):
+
+```bash
+cargo fmt --all -- --check
+cargo clippy --all-targets -- -D warnings
+cargo test
+```
+
+If any command is intentionally skipped, explain why.
+
+## Security Impact
+
+- New permissions/capabilities? (`Yes/No`)
+- New external network calls? (`Yes/No`)
+- Secrets/tokens handling changed? (`Yes/No`)
+- File system access scope changed? (`Yes/No`)
+- If any `Yes`, describe risk and mitigation:
+
+## Agent Collaboration Notes (recommended)
+
+- [ ] If agent/automation tools were used, I added brief workflow notes.
+- [ ] I included concrete validation evidence for this change.
+- [ ] I can explain design choices and rollback steps.
+
+If agent tools were used, optional context:
+
+- Tool(s):
+- Prompt/plan summary:
+- Verification focus:
+
+## Rollback Plan
+
+- Fast rollback command/path:
+- Feature flags or config toggles (if any):
+- Observable failure symptoms: