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/registry.go

@@ -0,0 +1,95 @@
+package build
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+)
+
+const registryFileName = "registry.gen.go"
+
+type registryRef struct {
+	Package string
+	Ident   string
+}
+
+type registryData struct {
+	Package          string
+	Module           string
+	Imports          []string
+	Models           []registryRef
+	Migrations       []registryRef
+	Commands         []registryRef
+	Jobs             []registryRef
+	AdminControllers []registryRef
+}
+
+func emptyRegistry(pkg, module string) registryData {
+	data := registryData{Package: pkg, Module: module}
+	data.prepare()
+	return data
+}
+
+func (d *registryData) prepare() {
+	d.Imports = uniqueLeafImports(d.Module, d.Models, d.Migrations, d.Commands, d.Jobs, d.AdminControllers)
+}
+
+func uniqueLeafImports(module string, groups ...[]registryRef) []string {
+	if module == "" {
+		return nil
+	}
+	seen := make(map[string]struct{})
+	var imports []string
+	for _, group := range groups {
+		for _, ref := range group {
+			if ref.Package == "" {
+				continue
+			}
+			path := module + "/" + ref.Package
+			if _, ok := seen[path]; ok {
+				continue
+			}
+			seen[path] = struct{}{}
+			imports = append(imports, path)
+		}
+	}
+	sort.Strings(imports)
+	return imports
+}
+
+func writeRegistry(pluginDir string, data registryData) error {
+	data.prepare()
+	src, err := renderStub("registry.go", data)
+	if err != nil {
+		return err
+	}
+	path := filepath.Join(pluginDir, registryFileName)
+	existing, err := os.ReadFile(path)
+	if err == nil && bytes.Equal(existing, src) {
+		return nil
+	}
+	if err != nil && !os.IsNotExist(err) {
+		return fmt.Errorf("build: read %s: %w", path, err)
+	}
+	tmp, err := os.CreateTemp(pluginDir, registryFileName+".*")
+	if err != nil {
+		return fmt.Errorf("build: create %s temp: %w", registryFileName, err)
+	}
+	tmpName := tmp.Name()
+	if _, err := tmp.Write(src); err != nil {
+		tmp.Close()
+		os.Remove(tmpName)
+		return fmt.Errorf("build: write %s temp: %w", registryFileName, err)
+	}
+	if err := tmp.Close(); err != nil {
+		os.Remove(tmpName)
+		return fmt.Errorf("build: close %s temp: %w", registryFileName, err)
+	}
+	if err := os.Rename(tmpName, path); err != nil {
+		os.Remove(tmpName)
+		return fmt.Errorf("build: rename %s: %w", registryFileName, err)
+	}
+	return nil
+}