feat(server): jj-native checkpoints and review diffs

[THEjacob1000]

What Changed

Checkpoints and review diffs for jj repos. A checkpoint is the commit id of @ at the end of a turn, pinned under refs/t3/ like git. Restore is jj restore --from, so change ids, bookmarks and other workspaces survive and jj undo takes it back. Review diffs run git plumbing against the colocated store so the diff parsers don't change.

Why

Bookmarks show up in the ref picker, jj commit drops a visible commit in the user's log every turn, and op ids stop resolving once the op log is pruned. A raw ref is durable, invisible and moves nothing. After a prune jj forgets the object but git still has it, so restore imports it back through a temp bookmark.

Stack

1. feat(vcs): jj driver, detection and process wrapper #11373
2. This PR
3. feat(server): route workflow, hosting glue, and discovery by vcs kind #11383
4. feat(web,mobile): jj terminology and shared vcs action logic #11384

GitHub can't create stacked PRs from a fork, so the diff carries the PRs below. Review the top commit only.

Manual evidence

Manually exercised in the installed Linux desktop app, build 2a249f8e503a (the complete stack), using disposable repositories. These captures verify the integrated stack, not each lower PR in isolation.

Two real agent turns produced distinct file diffs. The baseline and both turn checkpoints retain the same jj change ID; main stays on the original commit. A separate workspace also produced a turn diff. Working-copy and bookmark review were exercised after a selected-file commit.

Restore was rechecked in an isolated dev instance. Reverting the second turn restores the first checkpoint on disk and returns the removed prompt to the composer. Undo, pruned-object recovery, conflicts and large-file cases remain untested.

Two turn diffs, checkpoint identity and restore

restore-verified.mp4

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Evidence captured with Codex; test turns used GPT-6 Astra.

[t3dotgg]

Note

🤖 GPT-6 Astra responding on behalf of Theo

We're closing this as part of a cleanup of old PRs. Thank you for the time and care you put into this contribution.

The current comparison includes 1,482 files and 479,637 changed lines, with conflicts in 547 files against the target branch. Much of that comparison comes from diverged base history, beyond the change described here. The latest implementation commit is from 2026-09-20.

At this point, regenerating a focused patch against the current intended base would be easier to review and maintain than updating this PR. We appreciate the problem report and the work already done; this thread will remain available as a reference. If you revisit it, please confirm the current direction with us and link a fresh, focused PR back here.