Extract agent event loop

[NSIETeam]

Summary

• move top-level inbound consume/route/schedule loop into mira.agent.agent_event_loop
• keep AgentLoop.run as a compatibility wrapper
• preserve runtime control routing, priority command dispatch, automation deferral, pending-queue injection routing, dispatch task tracking, timeout compaction checks, and MCP cleanup

Local validation

• uv run python -m pytest tests/agent/test_task_cancel.py tests/agent/test_unified_session.py tests/agent/test_runner_injections.py tests/agent/test_auto_compact.py -q
• uv run ruff check mira/agent/agent_event_loop.py mira/agent/loop.py
• uv run mypy
• uv run python -m pytest tests/agent tests/tools/test_message_tool.py tests/command -q

Issue progress

• Contributes to issue 120 by reducing mira/agent/loop.py from 1303 to 1209 lines. It remains open because the target is <=800 lines.