Feature Request: Support Shift+Arrow and Ctrl+A text selection in prompt input

[mu88]

Describe the feature or problem you'd like to solve

Currently, the input line in Copilot CLI does not support standard GUI-style text selection shortcuts:

• Shift+Arrow keys (e.g. Shift+Left, Shift+Right, Shift+Home, Shift+End) do not select text — they are either ignored or move the cursor without selection
• Ctrl+A moves the cursor to the beginning of the line (readline behavior) instead of selecting all input text

This makes it impossible to select a portion (or all) of the input in order to copy, cut, or replace it using familiar keyboard-driven workflows. In contrast, mouse selection + Ctrl+Shift+C does work via Windows Terminal, but keyboard-only text selection is not available.

Note: This is distinct from existing issues about readline cursor navigation (e.g. #1069) — the request here is specifically for visual text selection support in the prompt input, similar to how text editors and modern terminal input fields behave.

Proposed solution

Support shift-based text selection in the prompt input field:

• Shift+Left / Shift+Right — extend selection by character
• Shift+Home / Shift+Ctrl+Left — extend selection to beginning
• Shift+End / Shift+Ctrl+Right — extend selection to end
• Ctrl+A — select all input text (with a visual highlight), rather than just moving the cursor

Selected text should be copyable via the platform copy shortcut and replaceable by typing.

Example prompts or workflows

1. Typed a long prompt and want to replace a specific phrase → Shift+click or Shift+arrow to select it, then type the replacement
2. Want to copy the entire current input to reuse it → Ctrl+A then Ctrl+C
3. Want to select from cursor to end of line to cut it → Shift+End, then Ctrl+X

Additional context

• OS: Windows
• Terminal: Windows Terminal
• Shell: PowerShell 7
• Related issue: Feature Request: Add Cut (Ctrl+X) and Undo (Ctrl+Z) keyboard shortcuts for text editing #1684 (Cut/Undo shortcuts) — text selection is a prerequisite for useful Cut support