feat(skills): Skill Contract v1, unity-cli skills lint, dual plugin distribution (#160)

[akiojin]

Summary

• Introduces Skill Contract v1 and a Rust-native unity-cli skills lint subcommand (22 rules R01..R22) to enforce it across all unity-* skills.
• Migrates all 13 existing unity-* skills to the contract and adds the shared references/runtime-checklist.md to each one.
• Packages the skills as a dual plugin for Claude Code (.claude-plugin/) and Codex CLI (new .codex-plugin/ + .agents/skills/ + .agents/plugins/marketplace.json) from a single source of truth.
• Moves gh-skills-sync out of the distributed plugin into dev-skills/, retargets its Claude Code symlink, and relocates Codex-only gh-* helpers from the legacy .codex/skills/ path to the official .agents/skills/ path.
• Replaces the legacy scripts/skill-eval/static-skill-contract-check.sh (686 lines of bash) with cargo run -- skills lint --severity error in both lint.yml and skill-routing-eval.yml.

Specs and rationale live in #160 (cross-linked from #145).

Skill Contract v1 (high level)

• description: ≤ 1024 chars, front-loads a metadata.triggers token in the first 250 chars, third-person imperative, both Use when and Do not use clauses, at least one sibling mentioned.
• frontmatter: required name, description, allowed-tools (Bash(unity-cli:*), Read, Grep, Glob + Edit,Write only for unity-csharp-edit), metadata.{author,version,category,triggers,siblings}; user-invocable: false only for foundation skills.
• SKILL.md body: ≤ 500 lines, required H2 sections in order Use When → Do Not Use When → Preferred Flow → Examples → References, 1-level reference nesting, no time-sensitive language.
• references: every skill must include references/runtime-checklist.md; reference files > 100 lines need a ## Table of Contents.
• cross-skill: sibling declarations must be bidirectional (R12); shared triggers require cross-listed siblings (R22).
• symlink integrity: .claude/skills/<name> and .agents/skills/<name> must resolve to the canonical source (R20 / R21).

See docs/skills.md for the full rule index and .claude-plugin/plugins/unity-cli/CONTRIBUTING.md for the new-skill checklist.

Test plan

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test --all-targets — 246 passed (22 new skill linter tests + 224 existing)
• cargo run -- skills lint --severity error — 13 skills, 0 violations
• Symlink audit: all 13 .claude/skills/unity-* and .agents/skills/unity-* canonicalize to .claude-plugin/plugins/unity-cli/skills/unity-*
• Plugin manifests: .claude-plugin/plugins/unity-cli/plugin.json and .codex-plugin/plugin.json both at 0.2.0; .agents/plugins/marketplace.json lists the Codex plugin
• .codex/skills/ removed, gh-skills-sync relocated to dev-skills/, Codex-only gh-* helpers moved to .agents/skills/
• CI wiring: lint.yml and skill-routing-eval.yml now invoke cargo run -- skills lint --severity error; legacy bash script deleted
• Reviewer: manual Codex CLI plugins install ./ check when convenient

Refs: closes work for #160, supersedes the skill-routing portion of #145.

🤖 Generated with Claude Code

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[akiojin]

Fixed the blocking items on this PR.

Done

• Merged the latest develop into this branch and resolved the .codex/skills -> .agents/skills rename conflict for unity-development-loop.
• Added coverage-focused tests in src/skills/loader.rs and src/skills/tests.rs to exercise the new skill-linter code paths that dropped Rust coverage.
• Normalized .claude-plugin/plugins/unity-cli/skills/unity-development-loop/SKILL.md and added .claude-plugin/plugins/unity-cli/skills/unity-development-loop/references/runtime-checklist.md so cargo run -- skills lint --severity error is clean again after the merge.

Verification

• cargo test --all-targets skills -- --nocapture
• cargo llvm-cov --all-targets --json --summary-only --ignore-run-fail --output-path /tmp/unity-cli-cov.json produced overall 90.14%
• cargo run -- skills lint --severity error

CI is running on commit 9e080d2.

[akiojin]

Follow-up

• Fixed the Rust formatter failure by applying cargo fmt --all to src/skills/tests.rs.
• CI is re-running on commit be89b6c.