Permissions for non-existing directories are handled incorrectly

### Describe the bug

When instructing Copilot to run `docker exec -w /my/workdir mycontainer my_command`, it asks for permission for directory /my/workdir.

This is questionable behavior, because this is not a directory for the local machine, it's a directory on the container, but that's a separate topic for discussion. Let's say I want to tell Copilot to always allow this directory and not ask me for permissions:
```copilot --model gpt-5-mini --add-dir=/my/workdir -p 'Run this command: `docker exec -w /my/workdir mycontainer my_command`'```

This command fails: "Error executing prompt: Error: Directory does not exist or cannot be accessed: /my/workdir"

I guess Copilot expects this directory to exist on the local machine? But again, this is not a local directory. So Copilot's behavior is inconsistent, confusing, and provides no alternatives besides --yolo

### Affected version

GitHub Copilot CLI 1.0.22.

### Steps to reproduce the behavior

```copilot --model gpt-5-mini --add-dir=/my/workdir -p 'Run this command: `docker exec -w /my/workdir mycontainer my_command`'```

### Expected behavior

Expected one of 2 things:
* /my/workdir would be added to allowed directories, despite not existing locally
* or, maybe permissions for /my/workdir shouldn't be required in the first place, since this is a directory on the container, not a local one

### Additional context

_No response_

Start by reproducing the documented `copilot --add-dir=/my/workdir` command with the Docker `exec -w` prompt. Trace the CLI's directory validation and permission handling for non-existent paths, then add coverage for the reported command and verify that the chosen expected behavior is consistent.

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