feat(skills): add github/gh-stack to SKILLS.txt (#166)

Add the github/gh-stack skill, a GitHub CLI extension for managing
stacked branches and pull requests. The repo exposes a single skill
(gh-stack), pinned explicitly per the SKILLS.txt convention.


Claude-Session: https://claude.ai/code/session_01PrhwKmHTXmU5pEKcDk3ouM

Co-authored-by: Claude <noreply@anthropic.com>

Author: shunkakinoki

SKILLS.txt

@@ -67,6 +67,9 @@ getsentry/skills agents-md,claude-settings-audit,code-review,code-simplifier,com
 # github/awesome-copilot (257 total) - keep essential dev/infra skills
 github/awesome-copilot chrome-devtools,conventional-commit,create-implementation-plan,create-readme,create-specification,dependabot,doublecheck,editorconfig,eval-driven-dev,git-commit,github-issues,make-repo-contribution,multi-stage-dockerfile,my-issues,my-pull-requests,playwright-generate-test,prd,pytest-coverage,refactor,refactor-plan,secret-scanning,security-review,typescript-mcp-server-generator,update-implementation-plan,webapp-testing
 
+# github/gh-stack (1 total) - keep all
+github/gh-stack gh-stack
+
 # google/skills (58 total) - keep all
 google/skills agent-platform-alert-configuration,agent-platform-deploy,agent-platform-endpoint-management,agent-platform-eval-flywheel,agent-platform-inference,agent-platform-migrate-from-ai-studio,agent-platform-model-registry,agent-platform-prompt-management,agent-platform-rag-engine-management,agent-platform-skill-registry,agent-platform-tuning,agent-platform-tuning-management,alloydb-basics,bigquery-ai-ml,bigquery-basics,bigquery-bigframes,bigtable-basics,cloud-run-basics,cloud-sql-basics,datalineage-bigquery-asset-impact-analysis,detection-engineering-coverage-evaluation,firebase-basics,gcloud,gemini-agents-api,gemini-api,gemini-interactions-api,gke-app-onboarding,gke-backup-dr,gke-basics,gke-batch-hpc,gke-cluster-creation,gke-compute-classes,gke-cost,gke-golden-path,gke-inference,gke-multitenancy,gke-networking,gke-observability,gke-reliability,gke-scaling,gke-security,gke-storage,gke-upgrades,google-agents-cli-onboarding,google-analytics-admin-api-basics,google-analytics-data-api-basics,google-cloud-networking-observability,google-cloud-recipe-auth,google-cloud-recipe-foundation-builder,google-cloud-recipe-onboarding,google-cloud-waf-cost-optimization,google-cloud-waf-operational-excellence,google-cloud-waf-performance-optimization,google-cloud-waf-reliability,google-cloud-waf-security,google-cloud-waf-sustainability,iam-recommendations-fetcher,workload-manager-basics