Detect Jujutsu repositories and show their diff, files and settings #22

Outcome

T3 recognises a Jujutsu repository. In a SlipJay workspace the review diff, file listing and ignored-file filtering work, Settings → Version Control shows Jujutsu as available (with SlipJay as a detail of the row and a read-only workspace-provider line in the project defaults), and "Initialize Git" no longer appears where a Jujutsu repository exists.

Settled constraints

• Implement the existing VCS driver contract for Jujutsu: repository detection, work-tree check, workspace file listing, ignored-path filtering, remotes and diff preview. Register it and make auto-detection pick Jujutsu when .jj exists, including colocated checkouts.
• Mark the Jujutsu row implemented. Detect SlipJay the way the panel already detects each tool's executable.
• Hide the "Initialize Git" action whenever any driver detects a repository.
• Verify with the shared driver contract harness.
• Colocated checkouts use Jujutsu; their branch strip may change, which is accepted.

Acceptance

• In a plain Jujutsu workspace (no .git), the review diff shows its changes, expanding unchanged lines works, file search omits ignored files, and no "Initialize Git" button appears.
• In a colocated checkout, the Jujutsu driver is the one selected.
• Settings → Version Control lists Jujutsu as available, with SlipJay noted when slipjay is installed, and the project defaults show the workspace provider.
• Git-only repositories behave exactly as before.

Non-goals

Checkpoints, status line and Land (later Tickets).