Serve the local JSON-RPC server in every interactive session #5414

Describe the feature or problem you'd like to solve

An interactive copilot window serves its JSON-RPC server on loopback only when it starts with --ui-server. --help does not list that flag, and no setting turns it on. So a tool on the same machine cannot reach a window that a person opened as plain copilot.

Proposed solution

• A setting (for example "uiServer": true in settings.json) or an environment variable that starts the embedded server in every interactive session.
• Document --ui-server, and write its port where a local client can find it (for example beside inuse.<pid>.lock).

Example prompts or workflows

1. From a phone, queue a follow-up into the window that is open on the laptop (session.send, mode: enqueue).
2. Steer a running turn from another app (session.send, mode: immediate).
3. Stop a runaway turn from another device (session.abort).

Additional context

Copilot CLI 1.0.109, macOS arm64. session.send, session.abort and session.model.switchTo work today over --ui-server. The request is only to make that server available in every interactive window, loopback only.

Related: #3301 asks for a browser UI. This request is smaller: the server that --ui-server already starts, in every window.