feat: add GitHub Releases publishing and installer download support

- Add publish-release.yml workflow triggered on v* tags or workflow_dispatch
- Update staging-build.yml to exclude v* tags from staging release job
- Update install.sh to download from GitHub Releases when no local package set
- Add -V/--version flag and download_tapa_package() with curl/wget fallback
- Add "Installing from Releases" section to README with one-liner
- Bump version to 0.1.20260319

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>