docs: update CI/CD documentation to reflect split ci and release workflows

[Rubyj]

Summary of Changes

Updates AGENTS.md and README.md to accurately reflect the current CI/CD setup after several iterations:

• File Structure sections in both files now list ci.yml, release.yml, and pull_request_template.md separately
• AGENTS.md CI/CD section rewritten to describe the two separate workflows: ci.yml (luacheck on PRs and main) and release.yml (luacheck + packager on v* tags, with permissions: contents: write)
• README.md Releases & CI updated to note luacheck runs on pull requests as well as main

Testing

Non-functional — docs only.

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