feat: channel plugin architecture with decoupled configs

- Add plugin discovery via Python entry_points (group: nanobot.channels)
- Move 11 channel Config classes from schema.py into their own channel modules
- ChannelsConfig now only keeps send_progress + send_tool_hints (extra=allow)
- Each built-in channel parses dict->Pydantic in __init__, zero internal changes
- All channels implement default_config() for onboard auto-population
- nanobot onboard injects defaults for all discovered channels (built-in + plugins)
- Add nanobot plugins list CLI command
- Add Channel Plugin Guide (docs/CHANNEL_PLUGIN_GUIDE.md)
- Fully backward compatible: existing config.json and sessions work as-is
- 340 tests pass, zero regressions

nanobot/channels/dingtalk.py

@@ -11,11 +11,12 @@ from urllib.parse import unquote, urlparse
 
 import httpx
 from loguru import logger
+from pydantic import Field
 
 from nanobot.bus.events import OutboundMessage
 from nanobot.bus.queue import MessageBus
 from nanobot.channels.base import BaseChannel
-from nanobot.config.schema import DingTalkConfig
+from nanobot.config.schema import Base
 
 try:
     from dingtalk_stream import (
@@ -102,6 +103,15 @@ class NanobotDingTalkHandler(CallbackHandler):
             return AckMessage.STATUS_OK, "Error"
 
 
+class DingTalkConfig(Base):
+    """DingTalk channel configuration using Stream mode."""
+
+    enabled: bool = False
+    client_id: str = ""
+    client_secret: str = ""
+    allow_from: list[str] = Field(default_factory=list)
+
+
 class DingTalkChannel(BaseChannel):
     """
     DingTalk channel using Stream Mode.
@@ -119,7 +129,13 @@ class DingTalkChannel(BaseChannel):
     _AUDIO_EXTS = {".amr", ".mp3", ".wav", ".ogg", ".m4a", ".aac"}
     _VIDEO_EXTS = {".mp4", ".mov", ".avi", ".mkv", ".webm"}
 
-    def __init__(self, config: DingTalkConfig, bus: MessageBus):
+    @classmethod
+    def default_config(cls) -> dict[str, Any]:
+        return DingTalkConfig().model_dump(by_alias=True)
+
+    def __init__(self, config: Any, bus: MessageBus):
+        if isinstance(config, dict):
+            config = DingTalkConfig.model_validate(config)
         super().__init__(config, bus)
         self.config: DingTalkConfig = config
         self._client: Any = None