Add an Outbox for completed agent artifacts

[OddballAI]

Summary

• Adds an Outbox directly beneath Inbox, listing explicitly completed agent deliverables newest first.
• Opens authenticated relay artifacts in the native default app and links each row back to its source conversation.
• Adds a buzz messages send --outbox contract so ordinary working attachments stay out, while completed files retain their original filename.
• Teaches managed agents to attach completed files with the Outbox marker.

Related issue

None found in an upstream issue/PR search.

Testing

• Desktop static checks and 5,570 unit tests
• buzz-cli: 369 tests
• buzz-acp: 817 tests
• Tauri workspace: 2,874 tests passed, 19 ignored
• Rust Clippy with warnings denied for CLI, ACP, and Tauri workspaces
• Production-shaped desktop E2E build
• Playwright smoke test covering agent-only filtering, one-click artifact open, source navigation, and rendered screenshot inspection