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/providers/test_openai_compat_timeout.py

@@ -29,7 +29,7 @@ def test_openai_compat_provider_sets_timeout_on_local_http_client() -> None:
     with (
         patch("nanobot.providers.openai_compat_provider.AsyncOpenAI") as mock_async_openai,
         patch(
-            "nanobot.providers.openai_compat_provider.httpx.AsyncClient",
+            "httpx.AsyncClient",
             return_value=sentinel.http_client,
         ) as mock_http_client,
     ):