feat(jj): add guided repository configuration

[knocking4thcylinder]

Problem

Jujutsu excludes new files above its snapshot size limit, which can leave them outside T3 checkpoints. Changing that limit and a repository's commit identity currently requires command-line configuration.

Change

Extend the guided repository configuration from the Git settings PR to Jujutsu. Web, desktop, and mobile can read effective and repository values, save repository overrides for author name, email, and snapshot.max-new-file-size, and reset each override. The snapshot limit accepts 1–4096 MiB or 0 for no limit.

This draft currently uses a preserved stack base containing Git configuration PR #13940 and jj support PR #13816. Those upstream PRs are independent of each other. This diff contains only the Jujutsu extension and will be rebased as the prerequisites land.

Verification

• Focused Git and Jujutsu configuration tests passed: 2/2 on the rebased branch.
• Targeted server, web, and mobile typechecks passed; focused lint and formatting passed.
• A real jj fixture excluded a 2 MiB new file at the default limit, then included it after raising the limit to 4 MiB. The test also covers unlimited (0) and Reset.
• In an isolated web client, saved a 5 MiB jj limit and verified the repository config, then Reset removed the override. On the rebased stack, switching from Git to jj settings cleared an unsaved Git author edit.

Native mobile could not be manually exercised because device access is disabled in this environment. Desktop uses the web surface.

UI evidence

GPT-6-Sol via the Codex harness in T3 Code.