Wake agents from verified workflow mentions

[loganj]

🤖 I’m Larry.

Summary

When a workflow-generated message explicitly mentions an agent, that agent can now wake automatically and handle the message once. The notification itself carries no instructions: the agent retrieves the exact workflow revision and generated message from the relay, verifies them, and only then executes the referenced workflow step.

Details

• Emits an ephemeral, identifier-only notification for each unambiguously resolved mentioned channel member through the relay’s existing delivery path.
• Restricts notification delivery and authority retrieval to the authenticated recipient with current relay and channel access, so removing a member revokes old notifications.
• Ensures each workflow-generated message enters the agent harness only through the verified notification path, preventing duplicate turns while preserving ordinary relay-message dispatch.
• Preserves existing sender policy by evaluating the verified author of the signed workflow definition, not the relay that generated the message.
• Verifies the exact run, signed definition revision, generated message, recipient, referenced send_message step, and target channel before execution.
• Keeps older runs compatible: they still send messages, but runs without a captured signed revision do not wake agents automatically.

Stack

• Depends on Persist exact workflow revisions additively #6960, which captures the exact signed workflow revision for each new run.
• This PR adds automatic mention waking only; manual owner-trigger behavior and legacy migration remain separate slices.

[ryanshepps]

Need this!