Expose "export transcript to $EDITOR" as a bindable keybindings.json action

Title: Expose "export transcript to $EDITOR" as a bindable keybindings.json action

Body:

Currently, viewing the conversation in an external editor requires two manual steps:

1. Ctrl+O (app:toggleTranscript) to enter the Transcript view
2. v to export the conversation to a temp file and open it in $EDITOR/$VISUAL

The v export-to-editor behavior in Transcript view is not exposed as a named action in keybindings.json's action registry (only transcript:toggleShowAll and transcript:exit are listed for the Transcript context). Because of that, there's no way to bind a single key or chord to go straight from anywhere to "transcript in my editor" — you always have to hit Ctrl+O first, see the transcript render, then press v.

Feature request: register the export action under a name like transcript:openInEditor (or similar) so it can be bound directly, e.g. as a single key or a chord (ctrl+o v), without requiring the intermediate toggle step to be a separate keypress.

Context: I use zellij + Helix as $EDITOR, and jump into the transcript in Helix often enough that the two-step flow feels redundant.