Extract process message state machine

[NSIETeam]

Summary

• move inbound-message session-key resolution, delivery setup, stream tracking, and state transition driving out of AgentLoop
• keep AgentLoop._process_message as a thin compatibility wrapper
• add focused tests for state-machine flow, stream merge_next forwarding, trace recording, and delivery-session mismatch

Local validation

• uv run python -m pytest tests/agent/test_process_message.py tests/agent/test_runner_injections.py tests/agent/test_stop_preserves_context.py -q
• uv run ruff check mira/agent/process_message.py mira/agent/loop.py tests/agent/test_process_message.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 1580 to 1451 lines. It remains open because the target is <=800 lines.