Speed up validation with path-aware jobs

[marcleblanc2]

Summary

• split reusable validation into path detection plus parallel GitHub Actions, Markdown, Python, and package jobs
• keep the existing required Validate package check as an aggregate gate
• skip irrelevant validation jobs on PRs based on changed files
• remove low-value actionlint/npm/pip cache overhead while keeping uv cache

Test plan

• /opt/homebrew/bin/actionlint
• git diff --check -- .github/workflows