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/channels/registry.py

@@ -1,5 +1,4 @@
 """Auto-discovery for built-in channel modules and external plugins."""
-
 from __future__ import annotations
 
 import importlib
@@ -51,21 +50,44 @@ def discover_plugins() -> dict[str, type[BaseChannel]]:
     return plugins
 
 
+def discover_enabled(
+    enabled_names: set[str],
+    *,
+    _names: list[str] | None = None,
+    _include_all_external: bool = False,
+) -> dict[str, type[BaseChannel]]:
+    """Return channels whose module names are in *enabled_names*.
+
+    Uses cheap ``pkgutil.iter_modules`` to list names, then imports only
+    those that match — skipping the heavy third-party SDK imports of
+    unneeded channels.
+    """
+    names = _names if _names is not None else discover_channel_names()
+    result: dict[str, type[BaseChannel]] = {}
+    for modname in names:
+        if modname not in enabled_names:
+            continue
+        try:
+            result[modname] = load_channel_class(modname)
+        except ImportError as e:
+            logger.debug("Skipping built-in channel '{}': {}", modname, e)
+
+    external = discover_plugins()
+    shadowed = set(external) & set(result)
+    if shadowed:
+        logger.warning("Plugin(s) shadowed by built-in channels (ignored): {}", shadowed)
+    if _include_all_external:
+        result.update({k: v for k, v in external.items() if k not in shadowed})
+    else:
+        result.update({k: v for k, v in external.items() if k not in shadowed and k in enabled_names})
+
+    return result
+
+
 def discover_all() -> dict[str, type[BaseChannel]]:
     """Return all channels: built-in (pkgutil) merged with external (entry_points).
 
     Built-in channels take priority — an external plugin cannot shadow a built-in name.
     """
-    builtin: dict[str, type[BaseChannel]] = {}
-    for modname in discover_channel_names():
-        try:
-            builtin[modname] = load_channel_class(modname)
-        except ImportError as e:
-            logger.debug("Skipping built-in channel '{}': {}", modname, e)
-
-    external = discover_plugins()
-    shadowed = set(external) & set(builtin)
-    if shadowed:
-        logger.warning("Plugin(s) shadowed by built-in channels (ignored): {}", shadowed)
-
-    return {**external, **builtin}
+    names = discover_channel_names()
+    return discover_enabled(set(names), _names=names, _include_all_external=True)