Feature Request: Persist /add-dir allowed directories across sessions

## Feature Request

### Summary
The `/add-dir` command adds directories to the allowed list for file access, but these permissions are **session-scoped only** and are lost when the session ends. There is no built-in way to persist allowed directories across sessions.

### Current Behavior
- `/add-dir ` adds a directory to the allowed list for the current session
- `/list-dirs` shows the allowed directories for the current session
- After restarting Copilot CLI (even with `/restart`), all added directories are gone

### Expected Behavior
Users should be able to mark directories as **permanently trusted**, so they appear automatically in `/list-dirs` on every new session without needing to re-add them each time.

### Proposed Solution
- Add a `/add-dir --permanent` (or `/add-dir --global`) flag to persist directories in `~/.copilot/config.json`
- Add a corresponding `/remove-dir ` command to revoke persistent access
- `/list-dirs` should show both session-scoped and persistent directories (with a label distinguishing them)

### Use Case
When working with directories on a Windows host via WSL (e.g. `/mnt/c/Users/paolo/git/myproject`), users need to re-add the same directories at the start of every Copilot session, which is repetitive and breaks workflow.

### Workaround
Currently none — `trusted_folders` in `config.json` serves a different purpose (workspace trust for MCP server loading) and does not affect `/list-dirs`.

Start by tracing the `/add-dir` and `/list-dirs` command entry points and the handling of `~/.copilot/config.json`. Check how `trusted_folders` is currently used without treating it as the same permission system. Done means persistent directories can be added, removed, and distinguished from session-scoped entries across restarts.

Rédigé par le modèle d'indexation à partir du texte de l'issue.