Subagents: shared durable agent runtime and named delegation

[dcramer]

Objective

Add durable subagents without creating a second agent runtime. A subagent is another Junior conversation with its own mailbox, lease, session log, execution profile, continuation slices, and result history.

Subagents run in independent queue/serverless invocations through the same continuation endpoint as root agents. Their main differences are lineage, execution profile, and final delivery back to a parent conversation.

Architecture intent

• conversationId remains the durable execution and history identity.
• Named agents reuse their own conversation; each delegated task has a separate invocation/result record.
• Model and reasoning configuration survive every continuation slice.
• Completion is pushed into the parent mailbox through automatic conversation-event subscription; status remains pullable for recovery.
• Parent lineage must not create a separate run path or broaden credentials, tools, or destination authority.

Delivery slices

• Refactor agent execution around a shared conversation runtime #876 - shared conversation runtime and durable execution profiles
• Make agent continuation and final delivery conversation-neutral #877 - conversation-neutral continuation and final delivery
• Generalize conversation event subscriptions for internal agent results #878 - provider-neutral conversation event subscriptions
• Add durable subagent conversations and independent queue execution #879 - durable child conversations and independent queue execution
• Expose spawnAgent and getAgentResult with named agent continuity #880 - spawnAgent, getAgentResult, and named continuity
• Add subagent context, lifecycle controls, and execution limits #881 - context, lifecycle controls, and execution limits
• Migrate advisor onto the shared subagent runtime and add behavior coverage #882 - advisor migration and behavior coverage

Completion gates

• Root and child agents use the same run, resume, mailbox, lease, and session-log abstractions.
• Child work survives timeout, worker loss, queue redelivery, and parent completion.
• One invocation produces one durable result and one deduplicated parent notification.
• Named agents preserve isolated conversation history and stable profiles.
• Integration tests cover persistence and recovery; evals cover delegation and result consumption.