Add Lua CI checks

[dbilici]

What changed

• add a GitHub Actions workflow for pushes to main, pull requests, and manual runs
• install Lua 5.1 and syntax-check every Lua source file
• run tests/sorting_spec.lua
• verify the version in modinfo.lua is represented in README, changelog, and Workshop metadata
• use read-only repository permissions, concurrency cancellation, and a ten-minute timeout

Why

Pack & Sort currently relies on manual release validation. This adds a fast automated gate for Lua syntax, pure sorting regressions, and release metadata drift while keeping in-game UI, multiplayer, dedicated-server, and Workshop checks manual.

Validation

• git diff --check
• workflow YAML parsed locally
• release metadata check passed for v0.8.7
• Lua execution is delegated to the first GitHub Actions run because Lua 5.1 is not installed locally