Parallel agents silently fill a Mac disk with SwiftPM .build dirs

When many coding agents run in parallel on one Mac (reviewers, mutation testers, worktrees), each swift test in a large package creates a 300 MB-3.5 GB .build, and mutation loops copy whole packages into /tmp. Free space can drop from GBs to 0 within an hour, at which point Claude Code itself cannot run any Bash (it needs to create an output file in its tmpdir), and agents stop with ENOSPC. Mitigations: tell every agent to delete .build and scratch exports right after use, run heavy builds on a remote host, check df -h /System/Volumes/Data before any build, and find leftovers with find <roots> -maxdepth 7 -type d -name .build -prune | xargs du -sh.