feat(01-02): implement layered compass config

- Merge plugin defaults, sorted YAML, SUMMER_ env, overrides and Set
- Persist atomically to env overrides; Reload clears runtime Set
- Discover HasConfig before Register and print layered hello values

examples/hello/plugins/base/plugin.go

@@ -1,10 +1,16 @@
 package base
 
 import (
+	"embed"
+	"io/fs"
+
 	"git.golem15.com/golem15/summercms/backpack"
 	"git.golem15.com/golem15/summercms/party"
 )
 
+//go:embed config
+var configFS embed.FS
+
 // Plugin is the golem15.hello plugin.
 type Plugin struct{}
 
@@ -14,6 +20,8 @@ func (p *Plugin) Requires() []string { return nil }
 func (p *Plugin) Register(*backpack.App) error { return nil }
 func (p *Plugin) Boot(*backpack.App) error     { return nil }
 
+func (p *Plugin) ConfigFS() fs.FS { return configFS }
+
 func init() {
 	party.Register(&Plugin{})
 }