- bin/platform.sh: single source of truth for platform differences —
  OS detection (NH_OS override for tests), PATH bootstrap without
  shadowing caller-injected commands, platform default paths, scheduler
  abstraction (systemd timer + systemctl <-> launchd plist + launchctl
  with named entry scripts), file-level disable on darwin (survives
  reboot, one-click recoverable), sed -i / GNU-sed shims, root check
  only on linux, conf loading (EnvironmentFile equivalent)
- bin/schedule.py: plist parse/write + auto-detect by filename
  (timer/plist/.disabled); schedule sources listing subcommand
- harvest/continue/daily-digest: drop hardcoded nvm//home/yichu paths,
  self-bootstrap via platform.sh, harvest now loads conf itself
  (run.sh wrapper no longer required)
- add-source/modify-source: fully cross-platform schedule management
  (plist generation + launchctl on darwin, timer + systemctl on linux)
- status.sh: cross-platform overview/detail/journal (launchd log on darwin)
- deploy/migrate: linux-only guard with pointer to setup.sh
- tests: env-synced platform simulation (NH_OS), fake launchctl suite,
  darwin plist/disable/enable roundtrip tests; new disable contract
  (unload + preserve times) replaces clear-times; 37 tests green