[BUG] Meta and output files written inside box directory are readable by sandboxed code

[ombagal777]

Description

The engine currently writes stdout/stderr and related metadata files into paths under the sandbox box directory (e.g., box/compile_stdout.txt, box/run_stdout.txt). Because these files live inside the isolate box filesystem, untrusted user code running in the sandbox can read them.

This is “not harmful but sloppy” for stdout/stderr, it is still a privilege‑leak pattern and could interact badly with future meta fields or sensitive headers.

Impact

• Sandbox code can read its own compile_stdout.txt and run_stdout.txt.
• If future changes store more sensitive information in these files (e.g., internal debug output, error traces with environment details), that information becomes visible to untrusted code.
• Breaks the principle of least privilege between engine internals and sandbox processes.

Expected Behavior

• Engine‑internal meta and output files are stored outside the box directory and are not directly readable by sandboxed processes.
• Only the engine process should have access to those files; user code should see them only via the API response.

Actual Behavior

• Stdout/stderr files are written inside the box (e.g., box/compile_stdout.txt).
• Sandbox code can open and read them.

Possible Solution

• Write stdout/stderr and meta files to a directory outside the box (e.g., /tmp/chadbox/<boxId>/...) and pass absolute paths to isolate using --stdout, --stderr, and meta flags.
• Ensure that only the engine process has filesystem permissions to read these paths.
• Keep the box directory limited to user‑controlled code and data only.