Let users interrupt a running turn to edit and re-run the previous prompt

## Ask

When a turn is running and it becomes clear the agent is heading down the wrong path (e.g. a mistake in the last prompt is sending the model off on a tangent), there's no one-step way to stop and correct course. The user wants an "edit" affordance that:

1. Stops the in-flight turn, and
2. Reopens the previous prompt in the input box for editing, then re-runs from the edited prompt.

Essentially "stop + edit-last-message + resend" as a single action, so users can redirect without waiting for a wrong turn to finish or retyping from scratch.

## Environment

Orbit 0.4.1, Linux x64.

Related: #20 (confirm scope upfront).

The issue names no files or tests. Start by locating the running-turn controls and prompt input, then trace how a turn is stopped and how the previous prompt is represented. Done means one action stops the current turn, restores the previous prompt for editing, and re-runs the edited prompt.

Written by the indexing model from the issue text.