Add repo-owned nanodot review skill

[hsliuustc0106]

Summary

• Make .agents/skills/nanodot-review the canonical review skill in this repository
• Preserve existing exact-head review guidance, read-only batch selector, grounding helpers, and eight pinned regression cases
• Move the helper/selector/corpus tests alongside the skill and run them in CI; replace personal-installer-only assertions with standalone bundle-copy coverage
• Remove dependence on a separately installed review skill and update invocation paths for this repository

Validation

• Skill creator quick validation: passed
• Offline skill suite: 44 tests passed, including four historical defect/clean pairs
• Main scaffold suite: 5 tests passed using an existing pytest environment with this checkout on PYTHONPATH
• These are packaging, helper and narrow regression checks, not a general review-quality benchmark or full unmerged MVP runtime validation

Draft only; no runtime implementation changes.