feat: add GitHub Actions workflow for automatic github/linguist updates

[jakubdomanski]

Closes: #466

Summary

This PR adds a GitHub Actions workflow that automatically updates the pinned github/linguist SHA on a weekly schedule - every Monday at midnight. I matched dependabot weekly runs instead of proposed monthly runs.

Why This Matters

See this comment #465 (review) and linked issue.

What The Workflow Does

1. Fetches the latest github/linguist SHA from the upstream repository
2. Compares it against the pinned SHA in internal/generate/knownfiles/main.go
3. If different, updates the SHA and regenerates pkg/filetype/known_files_gen.go
4. Creates a PR with the updated files (or skips if already up to date)
5. Cleans up the branch on failure scenarios

Test Results

• Already up to date: https://github.com/jakubdomanski/config-file-validator/actions/runs/25811048563/job/75827125491

• **PR already exists but is not merged yet **: https://github.com/jakubdomanski/config-file-validator/actions/runs/25810963926/job/75826828703

• Successful update run: https://github.com/jakubdomanski/config-file-validator/actions/runs/25808773491/job/75818941976

• Update PR: chore: update github-linguist SHA to e535c9adf5306132e9df0b75ffe1ce2679873fe8 jakubdomanski/config-file-validator#17

• Failure cleanup test: https://github.com/jakubdomanski/config-file-validator/actions/runs/25808659049/job/75818511199 - verified that the autopr/XXX branch is deleted when the workflow fails, tested with REPO > Settings > Actions > General > Allow GitHub Actions to create and approve pull requests = disabled

Please verify that REPO > Settings > Actions > General > Allow GitHub Actions to create and approve pull requests = enabled before merging.