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/tools/test_exec_platform.py

@@ -27,7 +27,7 @@ class TestBuildEnvUnix:
     def test_expected_keys(self):
         with patch("nanobot.agent.tools.shell._IS_WINDOWS", False):
             env = ExecTool()._build_env()
-        expected = {"HOME", "LANG", "TERM"}
+        expected = {"HOME", "LANG", "TERM", "PYTHONUNBUFFERED"}
         assert expected <= set(env)
         if sys.platform != "win32":
             assert set(env) == expected
@@ -53,7 +53,7 @@ class TestBuildEnvWindows:
 
     _EXPECTED_KEYS = {
         "SYSTEMROOT", "COMSPEC", "USERPROFILE", "HOMEDRIVE",
-        "HOMEPATH", "TEMP", "TMP", "PATHEXT", "PATH",
+        "HOMEPATH", "TEMP", "TMP", "PATHEXT", "PATH", "PYTHONUNBUFFERED",
         *_WINDOWS_ENV_KEYS,
     }