feat(tools/jujutsu): Jujutsu (jj) support for the local dev-loop

[potiuk]

Add tools/jujutsu/ so the local dev-loop skills can run on Jujutsu (jj) working copies. jj is Git-compatible at the storage layer but exposes a fundamentally different working model — no staging area, anonymous/auto-evolving changes, first-class conflict commits, and jj-style revsets instead of branches. The fix-workflow and code-review skills, which assume Git's branch/commit/git add model, would produce wrong or confusing steps on a jj repo even though the backing store is Git.

Capabilities to provide:

• Map the dev-loop primitives onto jj: jj new/jj describe for commit, jj diff, jj log, jj bookmark for branch-equivalent hand-off
• Translate the fix-workflow "create branch → commit → push" recipe into the jj change/bookmark model
• Detect a jj working copy and route through this shim instead of raw git
• Reuse the VCS-capability abstraction from tools/mercurial/

Why: jj adoption is growing fast among individual maintainers and at Google (its primary sponsor), and it's a common front-end over Git-hosted repos (GitHub/GitLab). Because it's Git-backed, an adopter can use jj locally while the forge stays GitHub — so this is low-risk, high-leverage support that several power-user maintainers would benefit from immediately.

Reference:

• Jujutsu: https://github.com/jj-vcs/jj
• Tutorial / mental model: https://jj-vcs.github.io/jj/latest/tutorial/
• Depends on the VCS abstraction from the Mercurial bridge issue