perf: optimize gateway cold start from ~6.9s to ~460ms (#3918)

Channel lazy load: discover_enabled() only imports enabled channel
modules instead of all 18 modules with heavy SDKs (telegram, discord,
slack, etc). discover_all() now delegates to discover_enabled().

Lazy OpenAI client: defer AsyncOpenAI() + httpx construction to
_ensure_client() with asyncio.Lock double-checked locking. openai
and httpx imports moved from module-level into _ensure_client().

Minor: lazy Nanobot/RunResult and CronService exports via __getattr__.

Benchmark: 6910ms → 460ms (-93.3%)

tests/channels/test_channel_plugins.py

@@ -180,7 +180,7 @@ async def test_manager_loads_plugin_from_dict_config():
     )
 
     with patch(
-        "nanobot.channels.registry.discover_all",
+        "nanobot.channels.registry.discover_enabled",
         return_value={"fakeplugin": _FakePlugin},
     ):
         mgr = ChannelManager.__new__(ChannelManager)
@@ -210,7 +210,7 @@ async def test_manager_propagates_groq_transcription_api_base_to_channels():
     )
 
     with patch(
-        "nanobot.channels.registry.discover_all",
+        "nanobot.channels.registry.discover_enabled",
         return_value={"fakeplugin": _FakePlugin},
     ):
         mgr = ChannelManager.__new__(ChannelManager)
@@ -246,7 +246,7 @@ async def test_manager_propagates_openai_transcription_api_base_to_channels():
     )
 
     with patch(
-        "nanobot.channels.registry.discover_all",
+        "nanobot.channels.registry.discover_enabled",
         return_value={"fakeplugin": _FakePlugin},
     ):
         mgr = ChannelManager.__new__(ChannelManager)
@@ -498,10 +498,8 @@ async def test_manager_skips_disabled_plugin():
         providers=SimpleNamespace(groq=SimpleNamespace(api_key="")),
     )
 
-    with patch(
-        "nanobot.channels.registry.discover_all",
-        return_value={"fakeplugin": _FakePlugin},
-    ):
+    ep = _make_entry_point("fakeplugin", _FakePlugin)
+    with patch(_EP_TARGET, return_value=[ep]):
         mgr = ChannelManager.__new__(ChannelManager)
         mgr.config = fake_config
         mgr.bus = MessageBus()