feat(04-01): scaffold Winter-shaped plugins with generated registry

Render plugin.go, routes.go, leaf packages and go.mod from embedded
templates, emit empty registry accessors, and define job, admin, lang
and mail capability contracts so a new plugin compiles before artifacts.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/build/stubs/registry.tmpl

@@ -0,0 +1,73 @@
+{{define "registry.go"}}// Code generated by summer make. DO NOT EDIT.
+
+package {{.Package}}
+
+import (
+	"git.golem15.com/golem15/summercms/bonfire"
+	"git.golem15.com/golem15/summercms/pact"
+	"github.com/go-gormigrate/gormigrate/v2"
+{{- range .Imports}}
+	{{printf "%q" .}}
+{{- end}}
+)
+
+func generatedModels() []any {
+{{- if .Models}}
+	return []any{
+	{{- range .Models}}
+		&{{.Package}}.{{.Ident}}{},
+	{{- end}}
+	}
+{{- else}}
+	return nil
+{{- end}}
+}
+
+func generatedMigrations() []*gormigrate.Migration {
+{{- if .Migrations}}
+	return []*gormigrate.Migration{
+	{{- range .Migrations}}
+		{{.Package}}.{{.Ident}}(),
+	{{- end}}
+	}
+{{- else}}
+	return nil
+{{- end}}
+}
+
+func generatedCommands() []bonfire.Command {
+{{- if .Commands}}
+	return []bonfire.Command{
+	{{- range .Commands}}
+		{{.Package}}.{{.Ident}}(),
+	{{- end}}
+	}
+{{- else}}
+	return nil
+{{- end}}
+}
+
+func generatedJobs() []pact.Job {
+{{- if .Jobs}}
+	return []pact.Job{
+	{{- range .Jobs}}
+		{{.Package}}.{{.Ident}}(),
+	{{- end}}
+	}
+{{- else}}
+	return nil
+{{- end}}
+}
+
+func generatedAdminControllers() []pact.AdminController {
+{{- if .AdminControllers}}
+	return []pact.AdminController{
+	{{- range .AdminControllers}}
+		{{.Package}}.{{.Ident}}(),
+	{{- end}}
+	}
+{{- else}}
+	return nil
+{{- end}}
+}
+{{end}}