Add a way to disable skills I don't use

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` (#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.

```yaml
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.