Rework skill validator script

[nirinchev]

This PR proposes the following changes:

• Moves validate-skills.sh script to tools so that it can more easily be accessed outside of github actions.
• Tweaks validate-skills.sh behavior:
  • Removes changed file logic - the script already runs very quickly and passing structural validation is a prerequisite for merging PRs to main, making the logic to only run on changed files unnecessarily complex for the value we're getting.
  • Adds validation that skill-validator is installed.
  • Decouple reporting from GHA, so that it can be used locally.
• Stub a basic CONTRIBUTING.md with instructions how to run the validate-skills script as well as the review-skill skill.

[Copilot]

Pull request overview

This PR reworks the skill validator script by moving it from .github/scripts/ to tools/ (making it accessible outside GitHub Actions), simplifying its behavior (removing git-diff-based changed-file logic in favor of always validating all skills), adding local usability improvements (tool availability check, environment-aware output), and adding a CONTRIBUTING.md with contributor documentation.

Changes:

• Replaces .github/scripts/validate-skills.sh (changed-files logic, CI-only) with tools/validate-skills.sh (validates all skills, works both locally and in CI)
• Updates .github/workflows/validate-skills.yml to call the new script location, removes paths: trigger filter and fetch-depth: 0 requirement
• Adds CONTRIBUTING.md with instructions for adding and testing skills

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
tools/validate-skills.sh	New unified validator script with local/CI environment detection, tool availability check, and clean output
.github/scripts/validate-skills.sh	Deleted — replaced by tools/validate-skills.sh
.github/workflows/validate-skills.yml	Updated to point to new script; removed changed-files path filter and full fetch-depth requirement
CONTRIBUTING.md	New contributor guide with skill creation and testing instructions

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

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[nirinchev]

I'm going to go ahead and merge this as there's a few other PRs that are blocked on it, but feel free to do a comprehensive review and I'll address the comments in a follow-up.