Feature request: Persistent directory allow list across sessions

## Summary

When starting a new Copilot CLI session, I'm repeatedly prompted to allow directories I've already approved in previous sessions. The directory permissions don't persist across sessions.

## Current behavior

Each new session requires re-approving directories via `/add-dir` or `/allow-all`, even though `permissions-config.json` already stores tool approvals per directory.

## Expected behavior

Directories that have been previously allowed should be remembered across sessions, either via:
- A persistent allow list in `~/.copilot/settings.json` or `permissions-config.json`
- An option to mark directories as 'always allowed'
- Auto-allowing directories that already have tool approvals configured

## Workaround

Running `/allow-all` at the start of each session, or launching from the target directory.

Start by tracing the /add-dir and /allow-all flows, then inspect how permissions-config.json and ~/.copilot/settings.json currently store directory or tool approvals. Define which existing approvals should persist across sessions and verify that an approved directory is not prompted for again when a new session starts.

Scritto dal modello di indicizzazione a partire dal testo della issue.