CBox review prompt context isolation on sandbox restart

Problem

When a cbox sandbox restarts (either by re-attaching to an existing tmux session or resuming over an existing worktree), it can inherit a stale .cbox/.review-prompt.md file left over from a previous review run. Claude reads this file on context switch, injecting unrelated review instructions into what should be a clean task execution session. This causes the sandbox agent to behave as if it's in review mode — ignoring its actual task prompt and producing confusing, off-task output. The one-shot review flow cleans up after itself, but neither sandbox resume path does, creating this contamination window.

Context

Possible Solutions

Plan

Implementation Progress

Implementation

Root cause

When a sandbox resumes (either by attaching to an existing tmux session or by recreating a session over an existing worktree), the .cbox/.review-prompt.md file was not cleaned up. This stale file could be read by Claude on context switch, injecting unrelated review prompt context into the sandbox session.

The one-shot review flow (_run_review_in_tmux) properly deleted the file after review completion or timeout, but neither sandbox resume path did.

Fix

Added _clean_stale_review_prompt(working_dir) helper in cli.py that removes .cbox/.review-prompt.md if it exists (using unlink(missing_ok=True)).

Called before sandbox start in all paths: 1. Attach to existing session (before sending any commands or attaching) 2. Resume from existing worktree (before starting a new container) 3. Default sandbox fresh start (before creating a new session)

Files changed

• libs/cbox/cbox/cli.py — added _clean_stale_review_prompt() helper and moved cleanup to run unconditionally before start/resume
• libs/cbox/test_review_prompt_isolation.py — 5 tests covering both resume paths, default sandbox attach/fresh-start, and no-file case

Risks and follow-ups

• No risk: unlink(missing_ok=True) is safe and idempotent.
• Follow-up: consider whether .cbox/reviews/ directory should also be cleaned on sandbox resume (lower priority, review outputs are useful for history).

Review Feedback

• [ ] Review cleared