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>

pact/capabilities.go

@@ -1,6 +1,7 @@
 package pact
 
 import (
+	"context"
 	"io/fs"
 	"net/http"
 
@@ -53,6 +54,53 @@ type HasModels interface {
 	Models() []any
 }
 
+// JobArgs is the typed payload a job worker receives. Kind identifies the
+// job so a worker can reject unexpected argument types.
+type JobArgs interface {
+	Kind() string
+}
+
+// Job is a unit of background work. Phase 11 adapts this onto River; the
+// interface itself must not import River.
+type Job interface {
+	Work(ctx context.Context, args JobArgs) error
+}
+
+// HasJobs is implemented by plugins that register background jobs.
+type HasJobs interface {
+	Jobs() []Job
+}
+
+// AdminController is the compile-time admin controller contract. Phase 9
+// grows the schema pipeline; ID, model name and YAML config directory are
+// enough for generated stubs to compile.
+type AdminController interface {
+	ID() string
+	ModelName() string
+	ConfigDir() string
+}
+
+// HasAdminControllers is implemented by plugins that register admin controllers.
+type HasAdminControllers interface {
+	AdminControllers() []AdminController
+}
+
+// HasLang is implemented by plugins that ship embedded translation YAML
+// under lang/<locale>/<group>.yaml.
+type HasLang interface {
+	LangFS() fs.FS
+}
+
+// HasMailTemplates is implemented by plugins that ship Winter-shaped mail
+// templates and layout aliases. MailTemplatesFS contains views/mail assets;
+// MailTemplates lists dotted template names; MailLayouts maps a short layout
+// name to the full dotted layout name.
+type HasMailTemplates interface {
+	MailTemplatesFS() fs.FS
+	MailTemplates() []string
+	MailLayouts() map[string]string
+}
+
 // OptionalMessage is a service an optional plugin may publish so other
 // plugins can integrate without importing that plugin's package.
 type OptionalMessage interface {
@@ -62,14 +110,10 @@ type OptionalMessage interface {
 // Future capability families are type-asserted when their first consumer
 // packages exist:
 //
-//	HasJobs
 //	HasListeners
-//	HasAdminControllers
 //	HasNavigation
 //	HasPermissions
 //	HasSchedule
-//	HasMailTemplates
-//	HasLang
 //
 // The kernel type-asserts HasConfig (party, before Register), HasCommands
 // (generated app main, after Boot), HasMigrations (lagoon migrate), and