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>

internal/submodule/provider.go

@@ -33,6 +33,42 @@ type Locator interface {
 	Dir(path string) string
 }
 
+// Restorer is implemented by providers that can make a repository (and all
+// submodules below it) exactly what git records. Path "" is the
+// superproject. Restore may clone or fetch objects that are missing, but must
+// never prompt the user.
+type Restorer interface {
+	// RestoreLoss reports what Restore would discard, read from the disk at
+	// this moment.
+	RestoreLoss(ctx context.Context, path string) (Loss, error)
+	Restore(ctx context.Context, path string) error
+}
+
+// Loss is what a restore discards. The zero Loss means nothing is lost.
+type Loss struct {
+	Files        int // changed, conflicted and untracked files and directories
+	Modules      int // repositories with a loss, the repository itself included
+	Operations   int // merges, rebases, cherry-picks, reverts in progress
+	Commits      int // commits that no branch, tag or remote ref reaches
+	Repositories int // untracked repositories with their own .git directory
+	Unknown      int // repositories whose state could not be read
+}
+
+// Empty reports whether nothing would be lost.
+func (l Loss) Empty() bool { return l == Loss{} }
+
+// Add sums two losses.
+func (l Loss) Add(o Loss) Loss {
+	return Loss{
+		Files:        l.Files + o.Files,
+		Modules:      l.Modules + o.Modules,
+		Operations:   l.Operations + o.Operations,
+		Commits:      l.Commits + o.Commits,
+		Repositories: l.Repositories + o.Repositories,
+		Unknown:      l.Unknown + o.Unknown,
+	}
+}
+
 // Change reports that the git metadata of a repository changed.
 type Change struct {
 	Path      string // "" is the superproject