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

首先跟踪 ! shell 模式下的 Tab 处理，以及切换到 Issues 视图的 keybinding；该 issue 未指定文件或测试。使用列出的路径、git checkout 和 src/ 补全工作流验证行为，并在 Tab 使用底层 shell 的正常补全而不是切换 CLI 视图时，视为工作完成。