feat(tools): introduce plugin-based tool discovery and runtime context protocol

This commit implements a progressive refactoring of the tool system to support
plugin discovery, scoped loading, and protocol-driven runtime context injection.

Key changes:
- Add Tool ABC metadata (tool_name, _scopes) and ToolContext dataclass for
dependency injection.
- Introduce ToolLoader with pkgutil-based builtin discovery and
entry_points-based third-party plugin loading.
- Add scope filtering (core/subagent/memory) so different contexts load
appropriate tool sets.
- Introduce ContextAware protocol and RequestContext dataclass to replace
hardcoded per-tool context injection in AgentLoop.
- Add RuntimeState / MutableRuntimeState protocols to decouple MyTool from
AgentLoop.
- Migrate all built-in tools to declare scopes and implement create()/enabled()
hooks.
- Migrate MessageTool, SpawnTool, CronTool, and MyTool to ContextAware.
- Refactor AgentLoop to use ToolLoader and protocol-driven context injection.
- Refactor SubagentManager to use ToolLoader(scope="subagent") with per-run
FileStates isolation.
- Register all built-in tools via pyproject.toml entry_points.
- Add comprehensive tests for loader scopes, entry_points, ContextAware,
subagent tools, and runtime state sync.

tests/agent/test_subagent.py

@@ -0,0 +1,30 @@
+"""Tests for SubagentManager."""
+
+from pathlib import Path
+from unittest.mock import MagicMock
+
+import pytest
+
+from nanobot.agent.subagent import SubagentManager
+from nanobot.bus.queue import MessageBus
+from nanobot.providers.base import LLMProvider
+
+
+@pytest.mark.asyncio
+async def test_subagent_uses_tool_loader():
+    """Verify subagent registers tools via ToolLoader, not hard-coded imports."""
+    provider = MagicMock(spec=LLMProvider)
+    provider.get_default_model.return_value = "test"
+    sm = SubagentManager(
+        provider=provider,
+        workspace=Path("/tmp"),
+        bus=MessageBus(),
+        model="test",
+        max_tool_result_chars=16_000,
+    )
+    tools = sm._build_tools()
+    assert tools.has("read_file")
+    assert tools.has("write_file")
+    assert tools.has("glob")
+    assert not tools.has("message")
+    assert not tools.has("spawn")