feat: add Homebrew cask for install

[jnahian]

Summary

Adds Homebrew support so Reader.md can be installed with brew.

• Casks/reader-md.rb — a Homebrew Cask pinned to the current release (v1.5.0) with the DMG's verified sha256. It installs Reader.md.app, marks auto_updates true so the app's own Sparkle updater owns upgrades (Homebrew won't replace a Sparkle-updated build), requires macOS 13+ (:ventura), and adds a github_latest livecheck plus a zap stanza for a clean uninstall.
• release.sh — after uploading the release, rewrites the cask's version and sha256 from the exact DMG just published and commits it (chore: update Homebrew cask …), so the tap tracks each release with nothing to bump by hand.
• README.md — new Install section documenting both the Homebrew path and the direct DMG download.
• release skill — notes the new automated cask step and adds it to the "what goes missing" table.

Because the repo isn't named homebrew-*, users tap it once with its explicit URL:

brew tap jnahian/reader.md https://github.com/jnahian/reader.md
brew install --cask reader-md

Related issue

Closes #

Testing

There's no test target; this change is packaging/docs only (no app runtime surface).

• ruby -c Casks/reader-md.rb — syntax OK
• bash -n release.sh — syntax OK
• Verified the cask's sha256 matches the published Reader.md.dmg by downloading it and running shasum -a 256 (cdd7b8dc…f18585)
• brew install --cask on a macOS machine (requires Homebrew; not available in CI env)

Checklist

• macOS 26-only APIs are guarded with a pre-26 fallback (deployment target is 13) — N/A, no Swift changes
• Imperative actions follow the token-bump pattern on AppState — N/A, no Swift changes
• Change is focused; matches surrounding code style

🤖 Generated with Claude Code

https://claude.ai/code/session_014pNnejYExL1PVYmSjkgANE

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