Add a keyboard modal for selecting and actioning structured reply units

Summary

Codex should support a keyboard-first modal selection mode for assistant replies, so users can navigate structured reply units and act on them without touching the mouse.

This is the keyboard companion to #14384.

The idea is to make reply structure directly operable:

• paragraphs remain whole
• list items remain whole
• tool calls / tool results can be selected as units
• code blocks can be selected as units

Problem

Right now, once a reply has been generated, the main path for reusing part of it is still too composer-bound:

• manual text selection
• mouse interaction
• retyping
• awkward clipboard workflows

That wastes a lot of structure that Codex already produced.

The terminal should let users break attention out of the composer and operate directly on reply objects with the keyboard.

Requested behavior

Add a modal reply-selection mode with keyboard navigation, for example:

• Alt+Up / Alt+Down to move selection across structured reply units
• selected units are visibly highlighted
• paragraphs remain whole instead of degenerating into line fragments
• reply units can include:
• paragraphs
• bullet items
• numbered items
• code blocks
• tool calls
• tool results

Suggested actions

Inside this modal:

• a = append selected unit(s) into the composer as quoted text (> ...) and remain in selection mode
• Enter = same as a, but return focus to the composer immediately
• Space = toggle selection for multi-select
• Shift+Up / Shift+Down = stretch selection range

Configurable action system

This gets much more powerful if actions are configurable instead of hardcoded.

For example, users should be able to bind keys to actions like:

• append_quote
• append_plain
• send_quote
• send_with_prefix
• pipe_to_command

Example configured actions:

• quote and append, then let me continue typing
• append with a prefix like fix this
• send immediately in chat
• pipe the selected text into a shell script

Example mental model:

• select one or more reply units
• hit a key
• Codex performs the configured action on that structured selection

Why this matters

This becomes an excellent Swiss-army-knife interaction surface.

It dramatically improves developer flow because users are less trapped inside the composer and can move their attention across multiple zones:

• the live thread
• structured prior reply content
• the composer
• external command/script actions

That makes Codex feel more like an operable control surface and less like a single text box.

Design notes

• this should be visually obvious when the mode is active
• selection boundaries should follow semantic reply structure, not wrapped terminal lines
• multi-selection should feel first-class, not bolted on
• actions should be configurable in both keys and behavior
• mouse and keyboard flows should feel like siblings, not separate products

Related issue

• Mouse-first version: #14384

Invitation

I’d especially encourage discussion on:

• better default keybindings
• additional action types
• good semantics for tool call / tool result selection
• multi-selection UX
• whether shell-command piping should be first-class or plugin-driven