Epic: agent module decomposition

[bug-ops]

Summary

agent/mod.rs is 2553 lines with 142 functions — the single largest complexity hotspot.

Child Issues

• Extract tool execution logic to agent/tool_execution.rs #648 Extract tool execution logic to agent/tool_execution.rs
• Extract message queue management to agent/message_queue.rs #649 Extract message queue management to agent/message_queue.rs
• Slim agent/mod.rs to main loop and dispatch (<500 lines) #650 Slim agent/mod.rs to main loop and dispatch (<500 lines)