[Mobile] Add “Edit from here” / checkpoint revert for user messages

[NANAMINER]

The web/desktop client already has an “Edit from here” action on eligible user messages, which lets you roll the thread back to that point and continue from there.

The mobile app currently has no equivalent action.

This is a significant workflow gap on mobile. A very common coding-agent workflow is:

1. Send a prompt.
2. Realize that the prompt was wrong, incomplete, or sent too early.
3. Stop the agent if necessary.
4. Go back to that user message.
5. Revert the conversation/checkpoint to that point.
6. Edit the prompt and send the corrected version.

On Android, steps 4–6 currently cannot be done from the T3 app. There is no “Edit from here” / revert action on user messages, so the only workaround is to switch to the web/desktop client.

Expected behavior:

• Add an “Edit from here” action to user messages on mobile, ideally in the long-press/message actions menu.
• Use the same checkpoint/conversation rollback behavior as the existing web implementation.
• Restore the selected user message into the composer so it can be edited and resent.
• Preserve attachments and composer context where possible.
• Make it available for any user message that is eligible for revert, not only the most recent message.

This should ideally be feature-parity with the existing web/desktop “Edit from here” button rather than a separate mobile-specific behavior.

Related:

• [Bug]: Cannot stop a running turn, edit the last prompt, and restart #11083 — Cannot stop a running turn, edit the last prompt, and restart
• [Feature]: Esc to pop a just-sent message back into the composer #6219 — Esc to pop a just-sent message back into the composer

The key difference from #6219 is that this request is not only about immediately undoing the most recently sent prompt. It is about exposing the existing checkpoint revert / “Edit from here” workflow in the mobile UI.

Setup:

• Android app
• Remote T3 server
• OpenCode provider
• Server v0.0.42

[zilverjk]

I have a working implementation of this for apps/mobile and would like explicit approval of the direction and scope before opening a PR, per CONTRIBUTING. (#15525 was closed as a duplicate of this discussion.)

Proposed scope, limited to bringing the existing web/desktop flow to native mobile:

• An undo arrow next to Copy on eligible sent prompts. Eligibility matches web: the provider supports conversation rollback, the message's run has a ready checkpoint, and the connection can operate the thread. Provider subagent threads are excluded. The arrow is disabled while the agent is working or messages are waiting to send.
• A native confirmation with "Revert and keep changes". "Revert files too" is only offered when the thread has its own worktree, because the server refuses file restore otherwise. Web offers it there too, and it fails.
• It uses the existing checkpoint.rollback command with turnCount, like web, then puts the prompt text and its image/file attachments back in the composer.
• Three pure helpers move from apps/web into packages/client-runtime, so both clients share one implementation: revert eligibility, recallable prompt text, and waiting for the rollback to land. Web behavior is unchanged.
• docs/user/composer.md no longer says the action is web/desktop only.

Checked so far: focused tests in client-runtime, mobile and web, plus typecheck and lint for those packages. I also tested manually on an iPad dev build against a dev server. The conversation rewinds and the prompt returns to the composer. On a worktree thread, "Revert files too" also restores the edited file to its earlier contents.

Two choices I'd like confirmed: sharing those helpers through client-runtime, and hiding "Revert files too" on mobile when the thread has no worktree. Does this direction and scope work for you?