Extract agent runner loop wrapper

[NSIETeam]

Summary

• move AgentLoop runner execution setup into mira.agent.agent_runner_loop
• keep AgentLoop._run_agent_loop as a compatibility wrapper for existing call sites
• preserve request context binding, workspace/file state scopes, pending injection drain, checkpoint persistence, hook wiring, sustained-goal handling, and max-iteration stream fallback

Local validation

• uv run python -m pytest tests/agent/test_loop_runner_integration.py tests/agent/test_runner_injections.py tests/agent/test_loop_progress.py tests/agent/test_loop_tool_context.py tests/agent/test_hook_composite.py -q
• uv run ruff check mira/agent/agent_runner_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 1451 to 1303 lines. It remains open because the target is <=800 lines.