feat: add input_copy and input_cut keybindings for keyboard-based text selection

[flexiondotorg]

Summary

Adds support for copying and cutting selected text in the OpenCode TUI input field using keyboard shortcuts, enabling fully keyboard-centric clipboard workflows.

Fixes #7516

Changes

• Add input_copy keybinding (default: Ctrl+Insert)
• Add input_cut keybinding (default: Shift+Delete)
• Implement event handlers in prompt component using existing Clipboard API
• Works with text selected via Shift+Arrow and other keyboard selection methods

Default Keybindings

The feature uses CUA-standard (Common User Access) keybindings:

• Ctrl+Insert for copy
• Shift+Delete for cut

Why CUA-standard?

• ✅ Universal compatibility: Works across all terminal emulators (not intercepted like Ctrl+Shift+C/V)
• ✅ Industry standard: From IBM/Windows/Office, familiar to many users
• ✅ Consistent: Aligns with existing input_paste default (Shift+Insert)
• ✅ No conflicts: Avoids terminal native shortcuts

Users can customize these via config if desired.

Implementation Details

• Uses TextareaRenderable.hasSelection() to detect active selections
• Uses TextareaRenderable.getSelectedText() to retrieve selected text
• Uses TextareaRenderable.deleteChar() for cut operation (deletes selection)
• Graceful error handling with console logging
• Follows established patterns from input_paste and input_clear

Testing

• ✅ Type checking passes (all packages)
• ✅ All tests pass (622/622)
• ✅ Manual testing: Copy and cut operations work correctly
• ✅ Edge case: No-op when no text is selected (graceful handling)

Related

• Issue: #7516
• Branch: flexiondotorg:input_copy
• Base: anomalyco:dev