reviewer sandbox: a fresh isolated tree per needs job #435

Follow-up from #428 (reviewer sandbox build lane).

What is missing

Jobs linked by needs share one worktree. Each job now starts from a verified clean clone with a fresh HOME, temp and cache root, and processes left running by the previous job are killed. Three things still carry over:

• state changed inside .git/ (refs, objects, index);
• processes that start their own session and so escape the group kill;
• host-level state that jobs share.

What the fix would do

• Give each job its own fresh clone from a read-only source that the host provides, such as a bare clone of the assigned commit, mounted read-only.
• Give each job a writable, exec-capable scratch area sized for node_modules. The /tmp tmpfs is noexec, so this needs a second host bind or an exec tmpfs.
• Discard the job's directory afterwards. Ideally the host creates one sandbox container per job, which also isolates processes.
• Then retire removeCreatedPaths and the cross-job "not fresh" refusal.

Acceptance

Job 1 changes a tracked file, changes .git state and leaves a detached background process. Job 2 sees none of them.

Refs #425, #428.