Extract loop construction helpers

[NSIETeam]

Summary

• move AgentLoop.from_config config mapping into mira.agent.loop_construction
• move default tool registration into the same construction helper module
• keep AgentLoop.from_config and AgentLoop._register_default_tools as compatibility wrappers

Local validation

• uv run python -m pytest tests/agent/test_self_model_preset.py::test_from_config_injects_default_preset tests/agent/test_self_model_preset.py::test_from_config_static_preset_loader_does_not_enable_hot_reload tests/agent/test_runtime_refresh.py tests/agent/test_auto_compact.py tests/agent/test_tool_loader_scopes.py -q
• uv run ruff check mira/agent/loop_construction.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 1209 to 1124 lines. It remains open because the target is <=800 lines.