Remove gstack bundled skill

[thesongzhu]

Summary

• remove the bundled managed-skills/gstack skill from Friday
• remove the active starter-skill test wording that described gstack-style starter skills
• keep UI interaction unclaimed unless a separate Friday-owned proof lane is approved

Verification

• npm test -- --run test/integration/skills/friday-skill-registry-lifecycle.test.ts
• npm test -- --run test/unit/skills/runtime/friday-starter-diagnosis-skills.test.ts test/unit/skills/runtime/friday-wave1-starter-skills.test.ts test/unit/skills/runtime/friday-wave23-starter-skills.test.ts
• npm run typecheck
• npm run lint (0 errors, existing warnings)
• npm run check:secret-patterns
• git ls-files | rg -i 'gstack' || true
• git grep -n -i 'gstack' -- . || true