Initial commit: unitdore scaffold + syncup/edit commands

- config: load/save/add unit, Unit struct
- runtime: podman + docker discovery and exists check
- cmd: syncup (discover + reconcile), edit, cobra root
- PLAN.md: full project plan

runtime/runtime.go

@@ -0,0 +1,33 @@
+package runtime
+
+// Container represents a discovered running container.
+type Container struct {
+	Name    string
+	Image   string
+	Command string
+	Runtime string // "podman" or "docker"
+}
+
+// Runtime is the interface all container runtimes must implement.
+type Runtime interface {
+	Name() string
+	ListRunning() ([]Container, error)
+	Exists(name string) (bool, error)
+}
+
+// Available returns all supported runtimes (callers skip those with no binary).
+func Available() []Runtime {
+	return []Runtime{&Podman{}, &Docker{}}
+}
+
+// Get returns a runtime by name ("podman" or "docker").
+func Get(name string) Runtime {
+	switch name {
+	case "podman":
+		return &Podman{}
+	case "docker":
+		return &Docker{}
+	default:
+		return nil
+	}
+}