fix: notarize and staple release DMGs so Gatekeeper accepts them

[mvanhorn]

Summary

In .github/workflows/release.yml, add a "Notarize DMG" step in the build-signed job immediately after "Create DMG" and before "Create ZIP archive" / "Calculate checksums" (ordering matters: stapling modifies the DMG, so the sha256 must be computed after stapling - the existing step order already ensures this). The new step, guarded by if [ -f "VibeProxy-${{ matrix.arch }}.dmg" ] to match the workflow's existing non-fatal DMG handling, runs `xcrun notarytool submit "VibeProxy-${{ matrix.arch }}.dmg" --apple-id ... --team-id ...

Why this matters

A user downloaded VibeProxy-arm64.dmg from the official releases and found that while the app bundle inside is correctly notarized and stapled (spctl --assess --type execute accepts it), the DMG itself is only codesigned, not notarized: spctl --assess --type open --context context:primary-signature rejects it with "Unnotarized Developer ID" and xcrun stapler validate confirms no ticket is stapled. The release notes claim "Code Signed & Notarized - No Gatekeeper warnings", so the DMG artifact contradicts the stated guarantee. Root cause is visible in .github/workflows/release.yml: the "Notarize App" step submits and staples only VibeProxy.app; the subsequent "Create DMG" step codesigns the DMG via create-dmg's --codesign flag but never submits the DMG to notarytool or staples it.

See #398.

Testing

• Happy path: tag-triggered release run produces VibeProxy-arm64.dmg and VibeProxy-x86_64.dmg that pass xcrun stapler validate and spctl --assess --type open --context context:primary-signature (verified in-workflow by the new validate/assess commands; workflow YAML validated locally with actionlint/yamllint since CI secrets cannot be exercised from a fork). - Edge case: create-dmg fails and no DMG exists (the step uses || true) - the notarize step's existence guard skips cleanly instead of erroring on a missing file. - Error path: notarytool returns a non-Accepted status for the DMG - the step prints the notarytool log for the submission id and exits 1, failing the release rather than shipping an unnotarized DMG. - Regression guard: sha256 files still match the final (stapled) DMG bytes because "Calculate checksums" runs after the new notarize/staple step.

Fixes #398