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%)

nanobot/cron/__init__.py

@@ -1,6 +1,18 @@
 """Cron service for scheduled agent tasks."""
 
-from nanobot.cron.service import CronService
 from nanobot.cron.types import CronJob, CronSchedule
 
 __all__ = ["CronService", "CronJob", "CronSchedule"]
+
+_LAZY = {"CronService": ".service"}
+
+
+def __getattr__(name: str):
+    module_path = _LAZY.get(name)
+    if module_path is None:
+        raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
+    from importlib import import_module
+    mod = import_module(module_path, __name__)
+    val = getattr(mod, name)
+    globals()[name] = val
+    return val