fix(sandbox): stop concurrent reviews from evicting each other's checkouts - #1348

🟢 Confidence 8/10 · likely safe to merge
Prune, timer and breaker semantics all check out by reading; only the container-side shell behaviour is unverified because verify:image needs Docker and was not run.
_{quality 100/100 · no findings · tests partial · risk medium · 2/2 new units observable}

Stops concurrent sandbox reviews from evicting each other: eviction now goes by last use with a 15-minute grace, a dead clone's scratch directory is pruned by pid, one Worker call is capped, and a per-turn breaker refuses a sandbox that already failed. Contained; safe to merge once the container image check runs.

• checkout.ts evicts by last use and keeps anything touched within SANDBOX_CHECKOUT_GRACE_MINUTES
• Clone scratch directories are named after the clone's pid and pruned only when that pid is gone
• CloudflareRepoSandbox caps one Worker call at the command's wall clock plus CALL_HEADROOM
• withSandboxBreaker refuses sandbox tools per repository for the rest of the turn

Before merge

• Manual · Run bun run --cwd apps/sandbox verify:image (needs Docker): the clone and prune shell scripts changed and their container behaviour is unverified

_{95bc070 · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}