Replace Woodpecker CI pipelines with Forgejo Actions workflows,
align Makefile, Containerfile and goreleaser config with the
patterns used in mattermost-marketplace-ng.
- Add .github/workflows/ci.yml with format, goreleaser-lint, lint, build jobs
- Add .github/workflows/release.yml with tag-triggered goreleaser release
- Update Makefile: standardized variables, auto-install golangci-lint, go fmt + go mod tidy
- Update Containerfile: multi-stage build with scratch for minimal image
- Update .goreleaser.yml: expanded changelog groups, explicit release config
- Update .gitignore: add build/, .env, coverage, AI tool dirs
- Remove .woodpecker/ directory