test(agent): expand coverage and refactor test structure

- Add 42 tests for ContextBuilder (context.py: 0→42 tests)
- Add 37 tests for SubagentManager lifecycle (subagent.py: 2→37 tests)
- Add 42 unit tests for AutoCompact in isolation
- Split monolithic test_runner.py (3313 lines) into 9 focused files:
  test_runner_core, test_runner_hooks, test_runner_errors,
  test_runner_safety, test_runner_persistence, test_runner_governance,
  test_runner_tool_execution, test_runner_injections,
  test_loop_runner_integration
- Add 3 config passthrough tests (temperature/max_tokens/reasoning_effort)
- Fix fragile patch.object(__init__) in test_stop_preserves_context
- Create shared conftest.py with make_provider/make_loop factories

Total: 934 tests passing, 0 regressions

tests/agent/conftest.py

@@ -0,0 +1,93 @@
+"""Shared fixtures and helpers for agent tests."""
+
+from __future__ import annotations
+
+from pathlib import Path
+from types import SimpleNamespace
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+from nanobot.agent.loop import AgentLoop
+from nanobot.bus.queue import MessageBus
+from nanobot.providers.base import LLMProvider
+
+
+def make_provider(
+    default_model: str = "test-model",
+    *,
+    max_tokens: int = 4096,
+    spec: bool = True,
+) -> MagicMock:
+    """Create a spec-limited LLM provider mock."""
+    mock_type = MagicMock(spec=LLMProvider) if spec else MagicMock()
+    provider = mock_type
+    provider.get_default_model.return_value = default_model
+    provider.generation = SimpleNamespace(
+        max_tokens=max_tokens,
+        temperature=0.1,
+        reasoning_effort=None,
+    )
+    provider.estimate_prompt_tokens.return_value = (10_000, "test")
+    return provider
+
+
+def make_loop(
+    tmp_path: Path,
+    *,
+    model: str = "test-model",
+    context_window_tokens: int = 128_000,
+    session_ttl_minutes: int = 0,
+    max_messages: int = 120,
+    unified_session: bool = False,
+    mcp_servers: dict | None = None,
+    tools_config=None,
+    model_presets: dict | None = None,
+    hooks: list | None = None,
+    provider: MagicMock | None = None,
+    patch_deps: bool = False,
+) -> AgentLoop:
+    """Create a real AgentLoop for testing.
+
+    Args:
+        patch_deps: If True, patch ContextBuilder/SessionManager/SubagentManager
+                    during construction (needed when workspace has no real files).
+    """
+    bus = MessageBus()
+    if provider is None:
+        provider = make_provider(default_model=model)
+
+    kwargs = dict(
+        bus=bus,
+        provider=provider,
+        workspace=tmp_path,
+        model=model,
+        context_window_tokens=context_window_tokens,
+        session_ttl_minutes=session_ttl_minutes,
+        max_messages=max_messages,
+        unified_session=unified_session,
+    )
+    if mcp_servers is not None:
+        kwargs["mcp_servers"] = mcp_servers
+    if tools_config is not None:
+        kwargs["tools_config"] = tools_config
+    if model_presets is not None:
+        kwargs["model_presets"] = model_presets
+    if hooks is not None:
+        kwargs["hooks"] = hooks
+
+    if patch_deps:
+        with patch("nanobot.agent.loop.ContextBuilder"), \
+             patch("nanobot.agent.loop.SessionManager"), \
+             patch("nanobot.agent.loop.SubagentManager") as MockSubMgr:
+            MockSubMgr.return_value.cancel_by_session = AsyncMock(return_value=0)
+            return AgentLoop(**kwargs)
+    return AgentLoop(**kwargs)
+
+
+@pytest.fixture
+def loop_factory(tmp_path):
+    """Fixture providing a factory for creating AgentLoop instances."""
+    def _factory(**kwargs):
+        return make_loop(tmp_path, **kwargs)
+    return _factory