Add a way to disable skills I don't use #1206

gstack ships ~30 skills, but most users only actively use a subset. The unused ones still load their descriptions into every Claude Code session, which costs tokens.

I'd like a way to tell gstack "skip these skills" — and have that survive gstack-upgrade.

What I tried:

• Deleting symlinks from ~/.claude/skills/ works once, but gstack-relink recreates them on the next upgrade.
• gstack-config doesn't have a key for this.
• npx gstack-skills (npx gstack-skills — standalone skill manager for gstack (upgrade + install without setup) #863) replaces the upgrade flow entirely, which is more than I want.

Idea: a disabled_skills list in ~/.gstack/config.yaml. gstack-relink skips anything in the list.

disabled_skills:
  - make-pdf
  - pair-agent
  - benchmark-models

Related: #886, #689, #766 — all about reducing per-session token overhead.

Happy to send a PR if this sounds reasonable.