Bug: Pressing Tab in prompt box with placeholder text should apply the placeholder text

Bug Description

When there is placeholder/suggestion text displayed in the prompt input box, pressing the Tab key does not apply the placeholder text. This is a common UX pattern (similar to shell tab-completion or IDE autocomplete) that users expect.

Expected Behavior

When placeholder text is shown in the prompt input (e.g., a suggested command or continuation), pressing Tab should insert that placeholder text into the prompt, allowing the user to then edit or submit it.

Actual Behavior

Pressing Tab either does nothing or performs a different action (like cycling through UI elements) instead of applying the placeholder text.

Steps to Reproduce

1. Open Claude Code CLI
2. Wait for or trigger placeholder/suggestion text to appear in the prompt box
3. Press Tab
4. Observe that the placeholder text is not applied

Environment

• Claude Code CLI
• Reported via /report-issue command

Additional Context

This is a common UX pattern seen in:

• Shell tab-completion (bash, zsh, fish)
• IDE autocomplete (VS Code, IntelliJ)
• Search engines (Google search suggestions)
• AI assistants with inline suggestions

Implementing this would improve the user experience by allowing quick acceptance of suggestions without needing to retype or copy the placeholder text.

Possible Solution

Add an event handler for the Tab key in the prompt input that, when placeholder text is present, inserts that text at the cursor position (or replaces the entire input with the placeholder text).