[FEATURE] Open $EDITOR (Ctrl-G) from the subagent prompt editor before dispatch

Summary

The main TUI input supports breaking out to $EDITOR (Ctrl-G) for long-form prompts. The subagent prompt input, shown after selecting an agent but before the agent is dispatched, does not. The only available workaround is to start the agent with a short prompt and then continue, which defeats the purpose of authoring a long, considered prompt up front.

Motivation

Subagent prompts are exactly where long-form authoring matters most: they have to be self-contained (the subagent has no prior context) and they often include file paths, line numbers, constraints, and acceptance criteria. The single-line-style box is too cramped for that, and pasting from an external editor loses the box-border characters (see #6151).

Proposal

Allow Ctrl-G (or the configured external-editor binding) to open the subagent prompt buffer in $EDITOR from the pre-dispatch input, mirroring the behavior already available in the main TUI prompt.

Related

• #6151: closed bug about cursor navigation in the subagent prompt editor; the request there also covered $EDITOR support.
• #18990: request to make the external editor configurable (main prompt).