feat(01-02): add optional plugin services and HasPlugin

- App-scoped typed Publish/Lookup with duplicate-provider errors
- Set plugin IDs before Register so HasPlugin sees the full set
- Greeter uses pact.OptionalMessage without importing optional

examples/hello/plugins/optional/plugin.go

@@ -0,0 +1,43 @@
+package optional
+
+import (
+	"embed"
+	"io/fs"
+
+	"git.golem15.com/golem15/summercms/backpack"
+	"git.golem15.com/golem15/summercms/pact"
+	"git.golem15.com/golem15/summercms/party"
+)
+
+//go:embed config
+var configFS embed.FS
+
+// Plugin is the golem15.optional plugin. Greeter may use it without importing this package.
+type Plugin struct{}
+
+func (p *Plugin) ID() string         { return "golem15.optional" }
+func (p *Plugin) Requires() []string { return nil }
+
+func (p *Plugin) Register(app *backpack.App) error {
+	if app == nil {
+		return nil
+	}
+	return app.Publish[pact.OptionalMessage](extra{})
+}
+
+func (p *Plugin) Boot(*backpack.App) error { return nil }
+
+func (p *Plugin) ConfigFS() fs.FS { return configFS }
+
+type extra struct{}
+
+func (extra) Message() string { return "hello-from-optional" }
+
+var (
+	_ pact.HasConfig       = (*Plugin)(nil)
+	_ pact.OptionalMessage = extra{}
+)
+
+func init() {
+	party.Register(&Plugin{})
+}