Better error messages for `-p` mode failing on permissions checks

### Describe the bug

Copilot fails to edit file with:

> Permission denied and could not request permission from user

even if I told it "--allow-all-paths".

### Affected version

0.0.355 Commit: 90c0eb793

### Steps to reproduce the behavior

```
$ echo "abc" > file.txt

$ copilot --allow-all-paths -p "Change all the letters 'a' to 'b' in the file 'file.txt'."
I'll read the file first to see its contents, then replace all 'a' letters with 'b'.

✓ Read file.txt (6 lines)

✗ Edit file.txt
Permission denied and could not request permission from user

Let me use bash to change the file contents:

✗ Replace all 'a' with 'b' in file.txt
$ sed -i '' 's/a/b/g' /Users/rolf/test/copilot/file.txt && cat /Users/rolf/test/copilot/file.txt
Permission denied and could not request permission from user

I'm unable to modify the file due to permission restrictions. The file appears to be read-only or protected. You may need to change the file permissions first using `chmod` or run the operation with appropriate permissions.

Total usage est: 1 Premium request
Total duration (API): 17s
Total duration (wall): 21s
Total code changes: 0 lines added, 0 lines removed
Usage by model:
claude-sonnet-4.5 56.0k input, 435 output, 0 cache read, 0 cache write (Est. 1 Premium request)
```

### Expected behavior

Copilot should be able to edit a file when I give it permission to do so.

### Additional context

[copilot.log.zip](https://github.com/user-attachments/files/23527219/copilot.log.zip)