verify-libghostty-vt-node skill: three deferred script fixes from PR 25 review #27

Follow-up from the review of #25 (the verify-libghostty-vt-node agent skill). Codex review round 4 on head 676de35 reported three P2 findings. I deferred them under the review cap: they affect only agents that run the skill, not package users.

1. scripts/launch.sh rebuilds libghostty-vt even when the caller supplies a prebuilt library through LIBGHOSTTY_VT_PREFIX, or through LIBGHOSTTY_VT_INCLUDE_DIR plus LIBGHOSTTY_VT_STATIC_LIB. It must use the supplied artifacts, or refuse these variables with a clear message. Review thread: chore: add the verify-libghostty-vt-node driving skill #25 (comment)
2. scripts/doctor.sh checks only which addon file node-gyp-build picks. It must also compare that file's sha256 with addonSha256 in instance.json, and fail when they differ. Review thread: chore: add the verify-libghostty-vt-node driving skill #25 (comment)
3. scripts/drive.sh runs without errexit and does not check its setup writes (the journey file, command.txt). It must stop before it runs node when one of these writes fails. Review thread: chore: add the verify-libghostty-vt-node driving skill #25 (comment)

Findings in launch.sh came in four rounds (6, then 1, then 6, then 3). Before another hardening round, consider simplifying it. One way is to support only the managed libghostty-vt build and the default mktemp RUN_ROOT.

Done when: all three behaviors are fixed, the skill's 18 journeys still pass, and doctor.sh fails on a swapped addon file.

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