feat(skills): add external mutation reviews

[tiammomo]

Closes #83

What changed

• Add a RepoSteward review skill for GitHub and other external state mutations without granting execution authority.
• Bind authorization to a complete immutable plan, applicable stable identities, expected versions, transition fingerprint, postcondition, actor, and policy digest.
• Require durable pre-call attempts, per-target intent and results, fresh preconditions, conditional writes, and explicit residual races.
• Separate attempt stage, remote outcome, and policy compliance; reconcile response loss, partial progress, ABA, and retries against authoritative state.
• Distinguish non-atomic batches from service-side atomic transactions whose per-target results must project one shared outcome.

Why

A remote call can succeed while its response is lost, and an object can change again before readback. Treating an exception or current-state mismatch as proof of no write creates false audit records and unsafe retries. The review workflow makes those guarantees and remaining races explicit while preserving all operation-specific gates.

Validation

• Skill Creator quick_validate.py: valid
• RepoSteward repository skill catalog: valid
• Independent unsafe-design and held-out atomic-batch/ABA/native-state forward tests
• Hardened verifier: 370 unit tests, Ruff lint and format checks, CLI smoke test, and package build passed
• GitHub CI: passed for the reviewed head

Review focus

• Exact authority and target provenance required before mutation
• Atomic-batch exception versus non-atomic partial progress
• completed, blocked, failed, and outcome_unknown semantic mapping
• ABA-safe reconciliation, idempotent replay, and public_write compatibility behavior
• No bypass of submit, merge, Issue promotion, branch cleanup, credentials, or human-review gates; Issue feat: 审计并清理终态 PR 的远端工作分支 #77 remains the native branch-cleanup state-machine work

Automation disclosure

An external coding workspace assisted with the implementation and tests. tiammomo reviewed the final diff, understands it, and takes responsibility for this contribution.