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.

nanobot/agent/tools/mcp.py

@@ -144,6 +144,8 @@ def _normalize_schema_for_openai(schema: Any) -> dict[str, Any]:
 class MCPToolWrapper(Tool):
     """Wraps a single MCP server tool as a nanobot Tool."""
 
+    _plugin_discoverable = False
+
     def __init__(self, session, server_name: str, tool_def, tool_timeout: int = 30):
         self._session = session
         self._original_name = tool_def.name
@@ -227,6 +229,8 @@ class MCPToolWrapper(Tool):
 class MCPResourceWrapper(Tool):
     """Wraps an MCP resource URI as a read-only nanobot Tool."""
 
+    _plugin_discoverable = False
+
     def __init__(self, session, server_name: str, resource_def, resource_timeout: int = 30):
         self._session = session
         self._uri = resource_def.uri
@@ -316,6 +320,8 @@ class MCPResourceWrapper(Tool):
 class MCPPromptWrapper(Tool):
     """Wraps an MCP prompt as a read-only nanobot Tool."""
 
+    _plugin_discoverable = False
+
     def __init__(self, session, server_name: str, prompt_def, prompt_timeout: int = 30):
         self._session = session
         self._prompt_name = prompt_def.name