Sandbox should create $TMPDIR directory automatically

Summary

When sandbox is enabled, Claude Code sets TMPDIR=/tmp/claude but does not create this directory. This causes tools like just (and any command that writes temp files) to fail with:

IO error while trying to create a temporary directory: No such file or directory (os error 2) at path "/tmp/claude/just-fFRcbs"

Expected behavior

The sandbox should ensure $TMPDIR exists before executing any commands, since it's the sandbox that sets this non-standard path.

Current workaround

Adding mkdir -p /tmp/claude as a SessionStart hook.

Environment

• macOS (Darwin 25.2.0, arm64)
• Claude Code with sandbox enabled