refactor(pairing): move /pairing from BaseChannel to CommandRouter

/pairing is now a first-class built-in command dispatched through
CommandRouter, just like /status, /model, /dream, etc.

Benefits:
- WebUI automatically shows /pairing in the slash command palette
  (because builtin_command_palette() feeds /api/commands).
- All channels (Telegram, Discord, WebSocket, etc.) use the same
  dispatch path for /pairing; no more channel-level interception.
- The command still only works for already-authorised users because
  is_allowed() gates message ingestion before the bus.

Changes:
- Add handle_pairing_command() to nanobot.pairing.store — pure
  function callable from CLI, CommandRouter, and tests.
- Add cmd_pairing to nanobot.command.builtin and register in
  BUILTIN_COMMAND_SPECS + register_builtin_commands().
- Remove BaseChannel._handle_pairing_command() and the /pairing
  interception logic from _handle_message().
- Clean up unused pairing imports from base.py.
- Add unit tests for handle_pairing_command and cmd_pairing dispatch.

nanobot/channels/base.py

@@ -11,14 +11,9 @@ from loguru import logger
 from nanobot.bus.events import InboundMessage, OutboundMessage
 from nanobot.bus.queue import MessageBus
 from nanobot.pairing import (
-    approve_code,
-    deny_code,
-    format_expiry,
     format_pairing_reply,
     generate_code,
     is_approved,
-    list_pending,
-    revoke,
 )
 
 
@@ -247,12 +242,6 @@ class BaseChannel(ABC):
                 )
             return
 
-        # Intercept /pairing slash commands before they reach the agent loop
-        parts = content.strip().split(None, 1)
-        if parts and parts[0] == "/pairing":
-            await self._handle_pairing_command(sender_id, chat_id, parts[1] if len(parts) > 1 else "")
-            return
-
         meta = metadata or {}
         if self.supports_streaming:
             meta = {**meta, "_wants_stream": True}
@@ -269,83 +258,6 @@ class BaseChannel(ABC):
 
         await self.bus.publish_inbound(msg)
 
-    async def _handle_pairing_command(
-        self, sender_id: str, chat_id: str, subcommand_text: str
-    ) -> None:
-        """Execute a ``/pairing`` slash command and reply directly to the user."""
-        parts = subcommand_text.split()
-        sub = parts[0] if parts else "list"
-        arg = parts[1] if len(parts) > 1 else None
-
-        if sub in ("list",):
-            pending = list_pending()
-            if not pending:
-                reply = "No pending pairing requests."
-            else:
-                lines = ["Pending pairing requests:"]
-                for item in pending:
-                    expiry = format_expiry(item.get("expires_at", 0))
-                    lines.append(
-                        f"- `{item['code']}` | {item['channel']} | {item['sender_id']} | {expiry}"
-                    )
-                reply = "\n".join(lines)
-
-        elif sub == "approve":
-            if arg is None:
-                reply = "Usage: `/pairing approve <code>`"
-            else:
-                result = approve_code(arg)
-                if result is None:
-                    reply = f"Invalid or expired pairing code: `{arg}`"
-                else:
-                    channel, sid = result
-                    reply = (
-                        f"Approved pairing code `{arg}` — "
-                        f"{sid} can now access {channel}"
-                    )
-
-        elif sub == "deny":
-            if arg is None:
-                reply = "Usage: `/pairing deny <code>`"
-            else:
-                if deny_code(arg):
-                    reply = f"Denied pairing code `{arg}`"
-                else:
-                    reply = f"Pairing code `{arg}` not found or already expired"
-
-        elif sub == "revoke":
-            if arg is None:
-                reply = "Usage: `/pairing revoke <user_id>` or `/pairing revoke <channel> <user_id>`"
-            elif len(parts) == 2:
-                reply = (
-                    f"Revoked {arg} from {self.name}"
-                    if revoke(self.name, arg)
-                    else f"{arg} was not in the approved list for {self.name}"
-                )
-            elif len(parts) == 3:
-                reply = (
-                    f"Revoked {parts[2]} from {arg}"
-                    if revoke(arg, parts[2])
-                    else f"{parts[2]} was not in the approved list for {arg}"
-                )
-            else:
-                reply = "Usage: `/pairing revoke <user_id>` or `/pairing revoke <channel> <user_id>`"
-
-        else:
-            reply = (
-                "Unknown pairing command.\n"
-                "Usage: `/pairing [list|approve <code>|deny <code>|revoke <user_id>]`"
-            )
-
-        await self.send(
-            OutboundMessage(
-                channel=self.name,
-                chat_id=str(chat_id),
-                content=reply,
-                metadata={"_pairing_command": True},
-            )
-        )
-
     @classmethod
     def default_config(cls) -> dict[str, Any]:
         """Return default config for onboard. Override in plugins to auto-populate config.json."""