ci: add skills-ref validate to lint job

[danieljohnmorris]

Summary

• Add the official agentskills.io reference validator (Python skills-ref, pip-installable from the agentskills repo skills-ref subdirectory) as a step inside the existing lint job.
• The previous skip was over-cautious: skills-ref ships a clean pyproject.toml and installs cleanly via pip install git+https://github.com/agentskills/agentskills.git#subdirectory=skills-ref.
• Catches Agent Skills spec drift on every PR (frontmatter shape, name format, description length, allowed-tools, optional fields).
• The existing npm-based skill-validate job is left in place; running both gives us cross-implementation coverage.

Test plan

• skills-ref validate skills/ilo/ locally — Valid skill: skills/ilo (exit 0)
• cargo fmt --all -- --check clean
• cargo clippy --workspace --all-targets -- -D warnings clean
• cargo build clean
• git diff --exit-code ai.txt clean
• CI green on PR (lint job picks up the new step)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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