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/tools/test_subagent_tools.py

@@ -17,7 +17,8 @@ async def test_subagent_exec_tool_receives_allowed_env_keys(tmp_path):
     """allowed_env_keys from ExecToolConfig must be forwarded to the subagent's ExecTool."""
     from nanobot.agent.subagent import SubagentManager, SubagentStatus
     from nanobot.bus.queue import MessageBus
-    from nanobot.config.schema import ExecToolConfig
+    from nanobot.agent.tools.shell import ExecToolConfig
+    from nanobot.config.schema import ToolsConfig
 
     bus = MessageBus()
     provider = MagicMock()
@@ -27,7 +28,7 @@ async def test_subagent_exec_tool_receives_allowed_env_keys(tmp_path):
         workspace=tmp_path,
         bus=bus,
         max_tool_result_chars=_MAX_TOOL_RESULT_CHARS,
-        exec_config=ExecToolConfig(allowed_env_keys=["GOPATH", "JAVA_HOME"]),
+        tools_config=ToolsConfig(exec=ExecToolConfig(allowed_env_keys=["GOPATH", "JAVA_HOME"])),
     )
     mgr._announce_result = AsyncMock()
 
@@ -125,8 +126,10 @@ async def test_spawn_tool_rejects_when_at_concurrency_limit(tmp_path):
 
     mgr.runner.run = AsyncMock(side_effect=fake_run)
 
+    from nanobot.agent.tools.context import RequestContext
+
     tool = SpawnTool(mgr)
-    tool.set_context("test", "c1", "test:c1")
+    tool.set_context(RequestContext(channel="test", chat_id="c1", session_key="test:c1"))
 
     # First spawn succeeds
     result = await tool.execute(task="first task")