ci: validate PKGBUILD on pull requests

[ulises-jeremias]

Problem

PKGBUILD changes can break silently without CI.

Proposed solution

Add workflow to validate PKGBUILD/.SRCINFO on PRs where feasible.

[ulises-jeremias]

Taking this one myself — Homebrew/AUR packaging work needs org-specific permissions (tap publish, AUR credentials, distribution secrets), so it is not a good fit for external contributors right now.

If you want to help with related docs/tests that do not require those secrets, ping me in Discord and we can carve out a safe slice.

[ulises-jeremias]

PKGBUILD validation failing on main since 2026-07-31 (chronic CI). Consolidating from daily briefing — needs triage: review PKGBUILD diff since last success. Leaving trace.