Add durable subagent conversations and independent queue execution

[dcramer]

Objective

Prove that a subagent is a normal Junior conversation with independent durable execution and serverless continuation.

Scope

• Add durable agent binding and invocation records.
• Create a child conversation, mailbox task, execution profile, and parent relationship.
• Run the child through the normal queue, lease, session log, timeout, and continuation lifecycle.
• Persist a terminal result and notify the parent conversation.

Completion

• Child work survives worker loss and serverless timeout.
• Completion produces one durable invocation result and one parent event.
• The proof works without model-facing subagent tools.