feat(ci): automate Homebrew formula update on release (#80)

Replaces the commented-out TODO homebrew section with a working
automation modeled on rtk-ai/vox's release pipeline.

On each release, the new homebrew job:
1. Downloads checksums.txt from the just-published release
2. Parses sha256 for all 4 platforms (macOS/Linux × Intel/ARM)
3. Generates the formula with correct version and hashes
4. Pushes to rtk-ai/homebrew-tap via GitHub API

Requires HOMEBREW_TAP_TOKEN org secret (already used by vox).

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>