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/manager.py

@@ -31,23 +31,29 @@ class ChannelManager:
         self._init_channels()
 
     def _init_channels(self) -> None:
-        """Initialize channels discovered via pkgutil scan."""
-        from nanobot.channels.registry import discover_channel_names, load_channel_class
+        """Initialize channels discovered via pkgutil scan + entry_points plugins."""
+        from nanobot.channels.registry import discover_all
 
         groq_key = self.config.providers.groq.api_key
 
-        for modname in discover_channel_names():
-            section = getattr(self.config.channels, modname, None)
-            if not section or not getattr(section, "enabled", False):
+        for name, cls in discover_all().items():
+            section = getattr(self.config.channels, name, None)
+            if section is None:
+                continue
+            enabled = (
+                section.get("enabled", False)
+                if isinstance(section, dict)
+                else getattr(section, "enabled", False)
+            )
+            if not enabled:
                 continue
             try:
-                cls = load_channel_class(modname)
                 channel = cls(section, self.bus)
                 channel.transcription_api_key = groq_key
-                self.channels[modname] = channel
+                self.channels[name] = channel
                 logger.info("{} channel enabled", cls.display_name)
-            except ImportError as e:
-                logger.warning("{} channel not available: {}", modname, e)
+            except Exception as e:
+                logger.warning("{} channel not available: {}", name, e)
 
         self._validate_allow_from()