[Feature Request] Add keyboard shortcut to select all text in prompt buffer

Proposed behavior
A dedicated shortcut (e.g. Ctrl+Shift+A) that selects all text in the current prompt buffer only — not the transcript above. Standard Cmd+C / Ctrl+C then copies the selection.

Use cases

• Copy a long prompt to paste elsewhere
• Replace the entire input wholesale
• Move the prompt to an external editor

Environment

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.116