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/channels/test_base_channel.py

@@ -85,51 +85,3 @@ async def test_handle_message_group_ignores_unknown() -> None:
 
     assert channel._sent == []
 
-
-@pytest.mark.asyncio
-async def test_handle_pairing_command_list(monkeypatch) -> None:
-    channel = _DummyChannel({"allowFrom": ["owner"]}, MessageBus())
-    monkeypatch.setattr(
-        "nanobot.channels.base.list_pending",
-        lambda: [
-            {
-                "code": "ABCD-EFGH",
-                "channel": "dummy",
-                "sender_id": "123",
-                "expires_at": 9999999999,
-            }
-        ],
-    )
-
-    await channel._handle_pairing_command("owner", "chat1", "list")
-
-    assert len(channel._sent) == 1
-    assert "ABCD-EFGH" in channel._sent[0].content
-
-
-@pytest.mark.asyncio
-async def test_handle_pairing_command_approve(monkeypatch) -> None:
-    channel = _DummyChannel({"allowFrom": ["owner"]}, MessageBus())
-    monkeypatch.setattr(
-        "nanobot.channels.base.approve_code",
-        lambda code: ("dummy", "123") if code == "ABCD-EFGH" else None,
-    )
-
-    await channel._handle_pairing_command("owner", "chat1", "approve ABCD-EFGH")
-
-    assert len(channel._sent) == 1
-    assert "Approved" in channel._sent[0].content
-
-
-@pytest.mark.asyncio
-async def test_handle_pairing_command_revoke(monkeypatch) -> None:
-    channel = _DummyChannel({"allowFrom": ["owner"]}, MessageBus())
-    monkeypatch.setattr(
-        "nanobot.channels.base.revoke",
-        lambda ch, sid: sid == "123",
-    )
-
-    await channel._handle_pairing_command("owner", "chat1", "revoke 123")
-
-    assert len(channel._sent) == 1
-    assert "Revoked" in channel._sent[0].content