feat(updater): migrate releases to GitHub with a legacy bridge

[ahilles107]

What

Move Gravity 0.12.4 update manifests and binaries to public GitHub Releases. New installations check GitHub directly. Add a separate compatibility Worker that proxies the old manifest path for existing installations, preserving manifest contents and updater signature verification.

The release workflow uploads all assets to a draft before publishing it as latest. It stops uploading new files to R2. Existing R2 objects and official signing/telemetry configuration remain intact.

Why

Public releases make duplicate binary storage unnecessary. Existing apps have the old manifest URL embedded, so the bridge must stay available for dormant installations. Its production route is activated only after the GitHub transition release is published and verified. Rollback instructions preserve historical download URLs and signing keys.

Closes USE-1590: https://linear.app/usebetterdev/issue/USE-1590/migrate-gravity-updates-to-github-releases-with-legacy-manifest-bridge

How tested

• Full RUSTUP_TOOLCHAIN=1.98.1 pnpm run verify, including Docker visual tests.
• Marketing tests cover byte-preserving proxy responses, header/query isolation, HEAD requests, upstream failures, invalid manifests, unsupported methods and unrelated paths.
• Wrangler bridge deployment dry run and isolated workers.dev deployment; the deployed proxy serves the existing release manifest.
• Notice generation refreshed input hashes with no dependency or notice-text changes.
• Transition validation uses the exact Tauri/updater versions from 0.12.3 for check/download/signature verification; signing/notarization, GitHub artifact checks and legacy-route activation follow release publication.

No data migration or user configuration change is required. Keep the legacy manifest bridge long-term; do not remove the R2 custom domain or existing versioned artifacts.

[linear-code]

USE-1590