Shell (!) mode: Tab should trigger normal terminal completion, not switch to Issues

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

When executing a shell command directly by prefixing it with  ! , pressing Tab does not behave like a normal terminal's tab-completion (completing file/directory paths, commands, or flags). Instead, it switches focus/navigation to the Issues view within the CLI. This breaks the expected muscle-memory of a shell prompt and makes it harder to quickly complete paths or commands while working in  !  shell mode.

### Proposed solution

When a command is prefixed with  !  (direct shell execution), pressing Tab currently gets intercepted by Copilot CLI's own UI (switching focus/view to Issues) instead of being passed through to the underlying shell for standard completion (file paths, commands, flags — like a normal terminal). Since  !  is meant to give a raw shell pass-through experience, Tab keystrokes while in that mode should be forwarded to the shell's native completion instead of being captured by the CLI's own keybindings.

### Example prompts or workflows

1. In the Copilot CLI, type  !cd ../../projects/my-  and press Tab — expected: shell completes the path to the matching directory, same as typing it directly in a terminal.
2. Type  !git chec  and press Tab — expected: shell completes to  git checkout  (or cycles matches), not a switch to the Issues panel.
3. Type  !ls src/  and press Tab to cycle through files/folders in  src/  — expected: standard filename completion, same as bash/zsh/PowerShell.

### Additional context

_No response_

Inizia tracciando la gestione di Tab nella modalità shell ! e il keybinding che passa alla vista Issues; l'issue non indica alcun file o test. Verifica il comportamento con i workflow per il percorso indicato, git checkout e il completamento di src/, e considera il lavoro completato quando Tab usa il normale completamento della shell sottostante invece di cambiare le viste della CLI.

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