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.

tests/pairing/test_store.py

@@ -88,6 +88,76 @@ class TestListPending:
         assert store.list_pending() == []
 
 
+class TestHandlePairingCommand:
+    def test_list_empty(self) -> None:
+        reply = store.handle_pairing_command("telegram", "list")
+        assert reply == "No pending pairing requests."
+
+    def test_list_pending(self) -> None:
+        store.generate_code("telegram", "123")
+        reply = store.handle_pairing_command("telegram", "list")
+        assert "Pending pairing requests:" in reply
+        assert "telegram" in reply
+        assert "123" in reply
+
+    def test_approve(self) -> None:
+        code = store.generate_code("telegram", "123")
+        reply = store.handle_pairing_command("telegram", f"approve {code}")
+        assert "Approved" in reply
+        assert "123" in reply
+        assert store.is_approved("telegram", "123") is True
+
+    def test_approve_invalid(self) -> None:
+        reply = store.handle_pairing_command("telegram", "approve BAD-CODE")
+        assert "Invalid or expired" in reply
+
+    def test_approve_no_arg(self) -> None:
+        reply = store.handle_pairing_command("telegram", "approve")
+        assert "Usage:" in reply
+
+    def test_deny(self) -> None:
+        code = store.generate_code("telegram", "123")
+        reply = store.handle_pairing_command("telegram", f"deny {code}")
+        assert "Denied" in reply
+        assert store.approve_code(code) is None
+
+    def test_deny_unknown(self) -> None:
+        reply = store.handle_pairing_command("telegram", "deny BAD-CODE")
+        assert "not found" in reply
+
+    def test_revoke_current_channel(self) -> None:
+        code = store.generate_code("telegram", "123")
+        store.approve_code(code)
+        reply = store.handle_pairing_command("telegram", "revoke 123")
+        assert "Revoked" in reply
+        assert store.is_approved("telegram", "123") is False
+
+    def test_revoke_other_channel(self) -> None:
+        code = store.generate_code("discord", "456")
+        store.approve_code(code)
+        # Two-arg form: first arg is channel, second is user
+        reply = store.handle_pairing_command("telegram", "revoke discord 456")
+        assert "Revoked" in reply
+        assert store.is_approved("discord", "456") is False
+
+    def test_revoke_unknown(self) -> None:
+        reply = store.handle_pairing_command("telegram", "revoke 999")
+        assert "was not in the approved list" in reply
+
+    def test_revoke_no_arg(self) -> None:
+        reply = store.handle_pairing_command("telegram", "revoke")
+        assert "Usage:" in reply
+
+    def test_unknown_subcommand(self) -> None:
+        reply = store.handle_pairing_command("telegram", "foo")
+        assert "Unknown pairing command" in reply
+
+    def test_default_to_list(self) -> None:
+        store.generate_code("telegram", "123")
+        reply = store.handle_pairing_command("telegram", "")
+        assert "Pending pairing requests:" in reply
+
+
 class TestStoreDurability:
     def test_corruption_recovery(self, tmp_path, monkeypatch) -> None:
         path = tmp_path / "pairing.json"