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/slack.py

@@ -13,8 +13,35 @@ from slackify_markdown import slackify_markdown
 
 from nanobot.bus.events import OutboundMessage
 from nanobot.bus.queue import MessageBus
+from pydantic import Field
+
 from nanobot.channels.base import BaseChannel
-from nanobot.config.schema import SlackConfig
+from nanobot.config.schema import Base
+
+
+class SlackDMConfig(Base):
+    """Slack DM policy configuration."""
+
+    enabled: bool = True
+    policy: str = "open"
+    allow_from: list[str] = Field(default_factory=list)
+
+
+class SlackConfig(Base):
+    """Slack channel configuration."""
+
+    enabled: bool = False
+    mode: str = "socket"
+    webhook_path: str = "/slack/events"
+    bot_token: str = ""
+    app_token: str = ""
+    user_token_read_only: bool = True
+    reply_in_thread: bool = True
+    react_emoji: str = "eyes"
+    allow_from: list[str] = Field(default_factory=list)
+    group_policy: str = "mention"
+    group_allow_from: list[str] = Field(default_factory=list)
+    dm: SlackDMConfig = Field(default_factory=SlackDMConfig)
 
 
 class SlackChannel(BaseChannel):
@@ -23,7 +50,13 @@ class SlackChannel(BaseChannel):
     name = "slack"
     display_name = "Slack"
 
-    def __init__(self, config: SlackConfig, bus: MessageBus):
+    @classmethod
+    def default_config(cls) -> dict[str, Any]:
+        return SlackConfig().model_dump(by_alias=True)
+
+    def __init__(self, config: Any, bus: MessageBus):
+        if isinstance(config, dict):
+            config = SlackConfig.model_validate(config)
         super().__init__(config, bus)
         self.config: SlackConfig = config
         self._web_client: AsyncWebClient | None = None