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

tests/test_matrix_channel.py

@@ -12,7 +12,7 @@ from nanobot.channels.matrix import (
     TYPING_NOTICE_TIMEOUT_MS,
     MatrixChannel,
 )
-from nanobot.config.schema import MatrixConfig
+from nanobot.channels.matrix import MatrixConfig
 
 _ROOM_SEND_UNSET = object()