gstack stays OFF by default (no per-skill symlink in skills/, zero context
cost). enable_skill now gains a gstack branch: a skill absent from skills/
and skills-disabled/ but present in the skills-external/gstack submodule is
symlinked in on demand when a profile lists it; disable_gstack_not_in()
parks it again on an unrelated profile.
This makes `set full` (which lists 35 gstack skills) work without 35 bogus
"missing — try: bash link.sh" warnings, without abandoning the OFF-by-default
policy. The old remedy message was wrong (link.sh never creates gstack
skills) and is replaced with submodule-aware messages.
Refs BDR-030.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0169vjUD1sP9Nx4ZiCa8wvAw
