chore: add GitHub Actions CI/CD, luacheck, and BigWigs packager

[Rubyj]

Summary of Changes

Adds CI/CD infrastructure so the addon can be linted automatically and distributed as a versioned zip via GitHub Releases, instead of manually zipping and sharing files.

• .github/workflows/release.yml — GitHub Actions workflow: luacheck on every push to main; luacheck + package + GitHub Release on v* tag push (uses built-in GITHUB_TOKEN, no secrets needed)
• .luacheckrc — Luacheck config whitelisting WoW API globals and addon-owned globals
• .pkgmeta — BigWigs packager metadata excluding dev-only files from the release zip
• .gitignore — Ignores .release/ build dir and local *.zip artifacts
• .github/pull_request_template.md — Project-specific PR template
• README.md — Updated Installation section and added Releases & CI section
• AGENTS.md — Added CI/CD section with instructions for future contributors

Testing

Non-functional change — no in-game behaviour is modified.

To verify the CI workflow after merge:

1. Confirm a luacheck run triggers on the merge commit to main
2. To test the full release flow, bump ## Version: in CooldownTracker.toc and push a tag:

   git tag -a v1.2.0 -m "Version 1.2.0"
   git push origin v1.2.0

3. Confirm a GitHub Release is created with a CooldownTracker-1.2.0.zip attached

Checklist

• Tested in-game on Midnight (Interface 120001)
• No new global variables introduced (or .luacheckrc updated if needed)
• No taint-unsafe patterns introduced (no runtime CreateFrame, SetParent, or SetScript on secure frames)
• Data.lua spell entries include both duration and defaultDuration
• New dev-only files added to .pkgmeta ignore list
• ## Version: in CooldownTracker.toc bumped if this is a release