`mcp_registry_list_tools` accepts a registry-qualified name as `server_id`, then advises an RPC no agent can call

What happens

On a live account run (core built from #6304 at 2abd462ec):

use_skill integrations/mcp_registry_list_tools {"server_id": "io.github.ryudi84/uuid"}
→ server_id=io.github.ryudi84/uuid is not connected; connect it first via `mcp_clients_connect`

Two separate problems in one answer:

1. A qualified name was accepted where a server id belongs. io.github.ryudi84/uuid is a registry-qualified name, not the connection's server_id. The tool took it, looked it up, and reported "not connected" — which is true of that string and false of the server, which was connected. The model then reasonably concluded it had to connect an already-connected server.
2. The remedy names an RPC method no agent can call. mcp_clients_connect is not a tool on any belt. Telling the model to "connect it first via mcp_clients_connect" is an instruction it cannot act on — the same failure class as #6302, where prose named tools the model did not have.

Fix shape

• Either resolve a qualified name to the connected server's id, or reject it as malformed and say what a server_id looks like. "Not connected" is the one answer that is actively misleading.
• Point the remedy at something the caller can actually do. In an orchestrator session after #6304 that is use_mcp_server; inside mcp_agent it is that agent's own connect tool.

Acceptance

A qualified name is either resolved or rejected as malformed, never reported as "not connected", and no error text names a method that is not callable from the session receiving it. Pinned by a test.

Found during the #6302 live acceptance run.