fix(workflows): authorize trusted owner-only agent wakeups

[JustinPerea]

Summary

• have the relay attach the authenticated workflow owner to relay-signed workflow messages
• authorize owner-only agent wakeups only after verifying the relay identity, event signature, workflow marker, and exact owner attribution
• fail closed for missing, duplicated, malformed, forged, or non-relay-signed attribution

Why

Hosted workflows are emitted by the relay, so owner-only agents currently reject scheduled workflow mentions even when the workflow owner is authorized. The Bookmarks workflow fired, but Bookmark Bee's ACP harness dropped the event before queuing a turn.

This keeps the agent owner-only. It does not broaden the agent to anyone.

Verification

• just ci passed on commit 046827c921d4e933b7e8d17e043d733bd3873ce0
• desktop frontend: 4,129 tests passed
• desktop/Tauri: 2,169 passed, 14 ignored
• Flutter analysis and tests passed
• workspace formatting, clippy, builds, and Rust package tests passed

The follow-up commit only updates implementation-notes.md; source is unchanged from the tested commit.

Deployment requirement

End-to-end acceptance requires both artifacts:

1. deploy the relay half so workflow events carry the authenticated owner attribution
2. release/install the ACP/Desktop half that verifies that attribution

Originating Buzz channel: 965eb307-5e3b-4f4e-a9c6-94323d98982d
Thread root: 12e4e065724f58cd54a03b5354aa4dc981260e9c5b18a9cb3a0275b4ce11be7b