Make the superproject selectable via the header

The header stands for the root repository: it is selected on startup,
shows the root's own state word, and the detail pane shows its state
like a submodule's (HEAD vs. upstream, changes, unpushed, leftover
submodule clones as untracked repositories). Submodules are ignored in
the root's changes; their rows show them.

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

internal/submodule/model.go

@@ -16,6 +16,7 @@ type Repo struct {
 	Ahead      int
 	Behind     int
 	FetchedAt  time.Time
+	Root       *Submodule // the superproject as a selectable node; Path is "", no Children
 	Submodules []*Submodule
 }
 
@@ -32,6 +33,9 @@ type Submodule struct {
 	FetchErr error   // set if the last fetch failed; not an issue
 }
 
+// IsRoot reports whether s is the superproject itself.
+func (s *Submodule) IsRoot() bool { return s != nil && s.Path == "" }
+
 // Presence describes whether the submodule exists on disk.
 type Presence int
 
@@ -171,6 +175,7 @@ func (c *Changes) Sort() {
 
 // Status is the full state of one submodule.
 type Status struct {
+	Root       bool // the superproject: the "pin" is its HEAD
 	Presence   Presence
 	Checkout   Checkout
 	Worktree   Worktree