Add staging release desktop recipe

[tlongwell-block]

Summary

• add just staging-release to build and open an optimized desktop bundle pointed at the staging relay
• give the staging bundle its own product name and app identifier (Buzz Staging, xyz.block.buzz.app.staging)
• keep mesh off by default, with just mesh=1 staging-release opt-in matching the existing convention

Validation

• just staging-release completed successfully and opened desktop/src-tauri/target/release/bundle/macos/Buzz Staging.app
• build output used pnpm build / vite build through pnpm tauri build, not tauri dev
• verified bundle metadata: CFBundleIdentifier=xyz.block.buzz.app.staging, CFBundleDisplayName=Buzz Staging, CFBundleName=Buzz Staging
• verified the staging relay URL is baked into the release binary: wss://sprout-oss.stage.blox.sqprod.co
• verified sidecars in Contents/MacOS/ are executable, including buzz, buzz-acp, buzz-agent, buzz-dev-mcp, and git-credential-nostr
• pre-commit hooks passed during commit; pre-push hooks passed during push