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/base.py

@@ -128,6 +128,11 @@ class BaseChannel(ABC):
 
         await self.bus.publish_inbound(msg)
 
+    @classmethod
+    def default_config(cls) -> dict[str, Any]:
+        """Return default config for onboard. Override in plugins to auto-populate config.json."""
+        return {"enabled": False}
+
     @property
     def is_running(self) -> bool:
         """Check if the channel is running."""