fix: cache host install mode too, guard mode-cache read under pipefail

Fixed

• Host installs never wrote the .coding-crew/dev-commands.json "mode" cache the previous
  release added for docker — only the docker branch of ensure-deps.sh called
  _merge_mode_cache, so a host project still paid detect-mode.sh's full Makefile make -n
  dry-run loop on every worktree/issue. ensure-deps.sh's MAIN_ROOT call now also caches
  host mode when resolution lands there.
• detect-mode.sh aborted under set -euo pipefail reading a dev-commands.json that has
  no "mode" field yet (e.g. one written before this cache existed): grep finds nothing,
  exits non-zero, and pipefail took the whole pipeline down with it. The sed stage now ends
  with || true so a cache miss falls through to the existing detection order instead of
  crashing.
  • registry.json: crew-afk 2.2.37 -> 2.2.38 (skills/crew-afk/scripts/ensure-deps.sh),
    dep-install 1.3.14 -> 1.3.15 (skills/dep-install/scripts/detect-mode.sh).