[FEATURE] Add Tab autocompletion for ! shell commands

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When using the ! prefix to run bash commands directly (e.g., !ls /path/to/dir), Tab autocompletion for file paths, folder names, and bash commands is not available.

Current Behavior:

• Typing !cd /Users/mad and pressing Tab does nothing
• No shell-style autocompletion is available in this mode

Proposed Solution

Expected Behavior:

• Tab should autocomplete file/folder paths similar to a real shell
• Ideally, command name completion would also work (e.g., !gre → !grep)

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

CLI commands and flags

Use Case Example

Use Case:
When running quick shell commands via !, having to type full paths manually is slow and error-prone. Users often need to switch to an external terminal just to get path completion, then copy/paste back into Claude Code.

Additional Context

_No response_