feat: add skill validation CI job

[CybotTM]

Summary

• Add skill validation job calling reusable workflow from skill-repo-skill
• Checks SKILL.md frontmatter, word count, composer.json, plugin.json, file presence

Test plan

• CI passes with new validation job

[gemini-code-assist]

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[Copilot]

Pull request overview

Adds a CI “Skill Validation” job to the existing lint workflow by calling a reusable workflow from netresearch/skill-repo-skill, to validate SKILL.md and related skill/package metadata.

Changes:

• Introduce a new validate job to run skill repository validation via reusable workflow.

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[Copilot]

This reusable workflow is referenced by branch name (@main). That makes CI behavior mutable and increases supply-chain risk; it also diverges from the rest of this workflow which pins actions by commit SHA. Pin the reusable workflow to a specific tag or commit SHA (and ideally document the version in a comment) so runs are reproducible.

Suggested change

	uses: netresearch/skill-repo-skill/.github/workflows/validate.yml@main
	# validate workflow v1.0.0
	uses: netresearch/skill-repo-skill/.github/workflows/validate.yml@0123456789abcdef0123456789abcdef01234567