M28-B: SubAgentManager core with in-process A2A channels

[bug-ops]

Parent: #709

Summary

Implement SubAgentManager in zeph-core that manages sub-agent lifecycle using in-process mpsc channels with A2A message semantics.

Scope

• SubAgentManager struct with spawn/cancel/collect/statuses methods
• SubAgentHandle with task state, JoinHandle, CancellationToken, status watch channel
• SubAgentChannel using tokio::sync::mpsc for A2A message passing
• A2aMessage enum wrapping zeph-a2a types (SendMessage, StatusUpdate, ArtifactUpdate, Cancel)
• Sub-agent execution loop: receive task, run LLM turns, send progress, return result
• Integration point in Agent (new subagent_manager field)
• Foreground-only execution (blocking spawn)

Acceptance Criteria

• SubAgentManager spawns a sub-agent that completes a simple task
• Cancel terminates a running sub-agent
• Status updates flow from sub-agent to manager via watch channel
• max_turns limit enforced (sub-agent stops after N turns)
• timeout_secs enforced via tokio::time::timeout
• Completed sub-agent result collected via collect()
• zeph-a2a types reused for all protocol messages