docs: real roadmap + install/download + Homebrew cask

[aymandakirgh]

Makes the README honest and sets up downloads. Stacks on #4.

• Roadmap rewritten release-by-release: v0.1 (shipped — foundation, AI command bar local+remote, Settings, Battery HUD, CI+release pipeline) ✅, v0.2 (media, calendar), v0.3 (file shelf, system HUD), v0.4+ (streaming, extensions, voice). Replaces the flat all-unchecked v0.1 list.
• Install section fixed — build-from-source now uses xcodegen generate (was the wrong open gh-notch.xcodeproj); download points at Releases with the honest unsigned-build right-click-Open note; Homebrew via tap.
• Homebrew cask at Casks/gh-notch.rb, ready for the first tagged release.
• "Free & open — forever" section: MIT, no ads, no telemetry, bring-your-own-AI-endpoint.
• Feature table marked as the v1.0 vision so it doesn't overclaim unbuilt features.

The README links work the moment a release exists — which happens automatically when a v* tag is pushed (see RELEASING.md / the release workflow from #4).

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