Add `go-api` template: Go + Chi + Postgres + Docker

[SubhanshuMG]

Summary

Add a production-ready Go API template to ForgeKit.

Stack

• Framework: Chi (lightweight, idiomatic Go router)
• Database: Postgres with sqlx
• Migrations: golang-migrate
• Container: Multi-stage Dockerfile, docker-compose with Postgres service
• CI: GitHub Actions matrix on Go 1.21 + 1.22
• Health check: GET /healthz endpoint
• Tests: go test ./... with coverage

Acceptance criteria

• forgekit new my-api --template go-api scaffolds the project
• docker compose up starts the app and Postgres
• CI passes on first push
• Template registered in registry.json
• Docs page added at docs/templates/go-api.md

See Contributing for the template format.