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/email.py

@@ -15,11 +15,41 @@ from email.utils import parseaddr
 from typing import Any
 
 from loguru import logger
+from pydantic import Field
 
 from nanobot.bus.events import OutboundMessage
 from nanobot.bus.queue import MessageBus
 from nanobot.channels.base import BaseChannel
-from nanobot.config.schema import EmailConfig
+from nanobot.config.schema import Base
+
+
+class EmailConfig(Base):
+    """Email channel configuration (IMAP inbound + SMTP outbound)."""
+
+    enabled: bool = False
+    consent_granted: bool = False
+
+    imap_host: str = ""
+    imap_port: int = 993
+    imap_username: str = ""
+    imap_password: str = ""
+    imap_mailbox: str = "INBOX"
+    imap_use_ssl: bool = True
+
+    smtp_host: str = ""
+    smtp_port: int = 587
+    smtp_username: str = ""
+    smtp_password: str = ""
+    smtp_use_tls: bool = True
+    smtp_use_ssl: bool = False
+    from_address: str = ""
+
+    auto_reply_enabled: bool = True
+    poll_interval_seconds: int = 30
+    mark_seen: bool = True
+    max_body_chars: int = 12000
+    subject_prefix: str = "Re: "
+    allow_from: list[str] = Field(default_factory=list)
 
 
 class EmailChannel(BaseChannel):
@@ -51,7 +81,13 @@ class EmailChannel(BaseChannel):
         "Dec",
     )
 
-    def __init__(self, config: EmailConfig, bus: MessageBus):
+    @classmethod
+    def default_config(cls) -> dict[str, Any]:
+        return EmailConfig().model_dump(by_alias=True)
+
+    def __init__(self, config: Any, bus: MessageBus):
+        if isinstance(config, dict):
+            config = EmailConfig.model_validate(config)
         super().__init__(config, bus)
         self.config: EmailConfig = config
         self._last_subject_by_chat: dict[str, str] = {}