Tab completion after "!" does not resolve PATH-based commands

### Describe the feature or problem you'd like to solve

When using the exclamation mark to run commands, it would be great if we could press Tab to autocomplete those commands from our PATH.

### Proposed solution

Add shell-style completion for the `!` command prefix, mirroring the normal REPL behavior.

**Logs / screenshot**

### Example prompts or workflows

Example demonstrating the issue (Ubuntu WSL 24.04.1):

1. Open Copilot CLI
2. Type: `!myscrip` and press Tab
3. Expected: completion to `!myscript.sh`
4. Actual: no completion occurs, stays as `!myscrip`

The same tab completion works in a regular bash prompt but fails within the Copilot CLI's `!` command prefix.

### Additional context

_No response_