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/discord.py

@@ -3,9 +3,10 @@
 import asyncio
 import json
 from pathlib import Path
-from typing import Any
+from typing import Any, Literal
 
 import httpx
+from pydantic import Field
 import websockets
 from loguru import logger
 
@@ -13,7 +14,7 @@ from nanobot.bus.events import OutboundMessage
 from nanobot.bus.queue import MessageBus
 from nanobot.channels.base import BaseChannel
 from nanobot.config.paths import get_media_dir
-from nanobot.config.schema import DiscordConfig
+from nanobot.config.schema import Base
 from nanobot.utils.helpers import split_message
 
 DISCORD_API_BASE = "https://discord.com/api/v10"
@@ -21,13 +22,30 @@ MAX_ATTACHMENT_BYTES = 20 * 1024 * 1024  # 20MB
 MAX_MESSAGE_LEN = 2000  # Discord message character limit
 
 
+class DiscordConfig(Base):
+    """Discord channel configuration."""
+
+    enabled: bool = False
+    token: str = ""
+    allow_from: list[str] = Field(default_factory=list)
+    gateway_url: str = "wss://gateway.discord.gg/?v=10&encoding=json"
+    intents: int = 37377
+    group_policy: Literal["mention", "open"] = "mention"
+
+
 class DiscordChannel(BaseChannel):
     """Discord channel using Gateway websocket."""
 
     name = "discord"
     display_name = "Discord"
 
-    def __init__(self, config: DiscordConfig, bus: MessageBus):
+    @classmethod
+    def default_config(cls) -> dict[str, Any]:
+        return DiscordConfig().model_dump(by_alias=True)
+
+    def __init__(self, config: Any, bus: MessageBus):
+        if isinstance(config, dict):
+            config = DiscordConfig.model_validate(config)
         super().__init__(config, bus)
         self.config: DiscordConfig = config
         self._ws: websockets.WebSocketClientProtocol | None = None