Add restore (x): make a repository exactly what git records

Restores the selection and all submodules below it recursively, asking
only if work would be lost. May clone/fetch missing objects (D23).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture.md

@@ -64,6 +64,13 @@ type Watcher interface {
 }
 ```
 
+```go
+type Restorer interface {
+    RestoreLoss(ctx context.Context, path string) (Loss, error) // what a restore would discard
+    Restore(ctx context.Context, path string) error              // may clone/fetch what is missing
+}
+```
+
 ```go
 type Locator interface {
     Dir(path string) string                    // absolute directory, "" if unknown