Allow editing the most recent user message by reverting the latest turn into the composer #762

Summary

T3 Code already supports reverting thread state to an earlier checkpoint. A lightweight follow-up would let the user edit the last prompt by:

• restoring the latest eligible user message back into the composer
• reverting the thread to the checkpoint immediately before that turn
• letting the user resend the adjusted prompt

Why issue-first

• This changes interaction flow, not just presentation.
• It affects thread history semantics and checkpoint expectations.
• It needs agreement on scope before opening a PR.

Proposed constraints

• Only allow editing the latest eligible user message.
• Reuse checkpoint revert instead of mutating historical messages in place.
• Rehydrate image attachments when possible.
• Keep the first PR scoped to the chat composer/timeline only.

Comparable examples

• Codex lets you revise the most recent prompt and continue from there.
• Other chat coding tools typically scope message editing to the latest user turn to avoid confusing timeline rewrites.

Suggested assets

• Before/after screenshots
• Short video showing: click edit -> composer restored -> resend