Define Agent Exchange trust and grant rules

[zaridan]

Define Agent Exchange trust and grant rules.

Goal:

Let operators decide which actors can receive or initiate Relay work, under what scope, and with what approval requirements.

Acceptance:

• Grants can scope access by actor, capability, conversation, mailbox, tag, boundary, or policy.
• Grants can require human approval before action.
• Grants can be revoked.
• Grants record who created them and why.
• Relay request creation and routing can check grants through public core logic.
• No first-party module gets private grant behavior.