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_loop_tool_context.py

@@ -6,6 +6,7 @@ import pytest
 from nanobot.agent.loop import AgentLoop
 from nanobot.bus.queue import MessageBus
 from nanobot.providers.base import LLMResponse, ToolCallRequest
+from nanobot.agent.tools.context import RequestContext
 
 
 class _ContextRecordingTool:
@@ -15,18 +16,12 @@ class _ContextRecordingTool:
     def __init__(self) -> None:
         self.contexts: list[dict] = []
 
-    def set_context(
-        self,
-        channel: str,
-        chat_id: str,
-        metadata: dict | None = None,
-        session_key: str | None = None,
-    ) -> None:
+    def set_context(self, ctx: RequestContext) -> None:
         self.contexts.append({
-            "channel": channel,
-            "chat_id": chat_id,
-            "metadata": metadata,
-            "session_key": session_key,
+            "channel": ctx.channel,
+            "chat_id": ctx.chat_id,
+            "metadata": ctx.metadata,
+            "session_key": ctx.session_key,
         })
 
     async def execute(self, **_kwargs) -> str:
@@ -37,6 +32,10 @@ class _Tools:
     def __init__(self, tool: _ContextRecordingTool) -> None:
         self.tool = tool
 
+    @property
+    def tool_names(self) -> list[str]:
+        return ["cron"]
+
     def get(self, name: str):
         return self.tool if name == "cron" else None