feat: OpenRC and runit init backends behind an Init interface

Extract init-neutral unit resolution into service/, add initsys.Init with systemd, openrc and runit backends, selected by 'init:' config or autodetect. OpenRC and runit are root only; rootless units run as system services as that user. Add docs and generated examples for all three.

cmd/start.go

@@ -5,7 +5,7 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/warkanum/unitdore/config"
-	"github.com/warkanum/unitdore/systemd"
+	"github.com/warkanum/unitdore/initsys"
 )
 
 var startCmd = &cobra.Command{
@@ -32,13 +32,17 @@ func runStart(cmd *cobra.Command, args []string) error {
 	}
 
 	prefix, suffix := cfg.Prefix, cfg.Suffix
+	ini, err := initsys.New(cfg)
+	if err != nil {
+		return err
+	}
 
-	if !systemd.IsInstalled(*u, prefix, suffix) {
+	if !ini.IsInstalled(*u, prefix, suffix) {
 		return fmt.Errorf("%s is not installed — run 'unitdore install' first", name)
 	}
 
-	fmt.Printf("  ▶ starting %s...\n", systemd.ServiceName(*u, prefix, suffix))
-	if err := systemd.Start(*u, prefix, suffix); err != nil {
+	fmt.Printf("  ▶ starting %s...\n", ini.ServiceName(*u, prefix, suffix))
+	if err := ini.Start(*u, prefix, suffix); err != nil {
 		return err
 	}
 	fmt.Printf("  ✓ started: %s\n", name)