M28-E: Background sub-agent execution

[bug-ops]

Parent: #709

Summary

Support background (non-blocking) sub-agent execution, allowing the main agent to continue interacting with the user while sub-agents work concurrently.

Scope

• Background spawn: returns immediately, sub-agent runs on separate tokio task
• Concurrent sub-agent limit (max_concurrent in SubAgentManager)
• Status polling from main agent loop
• Result notification when background sub-agent completes
• Cleanup on main agent shutdown (cancel all background sub-agents)

Acceptance Criteria

• Background sub-agent does not block main agent conversation
• max_concurrent enforced (spawn returns error if limit reached)
• Main agent can poll status of background sub-agents
• Completion notification delivered to user
• All background sub-agents cancelled on main agent shutdown
• CancellationToken propagated correctly to background tasks