Promotes docker-prep.sh to canonical home in common-plat with full Phase B
hardening from the docker-build-optimization-roadmap:
- B1: --dry-run mode (lists actions, no side effects)
- B2: idempotency guard (refuses to run if *.bak exists, --force to bypass)
- B5: trap-based auto-restore on error (--keep to disable)
- B6: standardized header + usage block
- B7: canonical home + sync + drift-check (mirrors npmrc.template pattern)
- B8: --strip-overrides for safety-net cleanup
- New: --check mode for CI-friendly state verification
- New: auto-discovers package.json files with @bytelyst/* deps
- New: portable sed -i (BSD on macOS, GNU on Linux)
- New: preserves .docker-deps/.gitkeep on clear (fixes earlier regression)
- New: 2 small JS helpers (_docker-prep-*.js) avoid bash 3.2 heredoc quirks
Verified on clock + peakpulse: dry-run, pack, check, idempotency guard,
restore, and post-restore git status all clean.
