[F029] websocket: remove broken /api/permissions/check loopback from channel stream

channel_stream called validate_channel_access, which HTTP-loopbacked to
GET /api/permissions/check — a route that does not exist. Every call 404'd
-> False -> the channel stream closed with WS_1008_POLICY_VIOLATION for
EVERY client, so the real-time channel stream was dead. Removed the
function, its call site, and the now-unused httpx + settings imports.

Post-F004 the panel-token gate is the channel-stream authorization (the
CEO panel is the sole WS client and may view every channel), so the
broken loopback is removed rather than replaced with an in-process check
the CEO always passes. The legitimate enforcement.validate_channel_access
(slugs, in-process static ACL) is a different function and is untouched.

F027 is resolved-by-F004 (no code change): all three per-agent streams
gate on _require_panel_token first, so only the authorized CEO panel can
connect — 'any viewer subscribes to any target' is closed.

TDD; ruff/mypy clean; 530 unit/api+enforcement+RBAC tests green.

tests/unit/api/test_websocket_handler_cleanup.py

@@ -127,9 +127,6 @@ async def test_channel_stream_disconnects_on_non_disconnect_exception(
     mgr = ConnectionManager()
     ws = _mock_ws_for_receive(RuntimeError("anyio closed"))
     ws.query_params = {"agent_id": str(agent_id)}
-    monkeypatch.setattr(
-        "roboco.api.websocket.validate_channel_access", AsyncMock(return_value=True)
-    )
     monkeypatch.setattr("roboco.api.websocket.manager", mgr)
 
     with pytest.raises(RuntimeError):