feat(tui): workflow-start intent input box wired to StartSession.input

apps/tui-go/internal/app/model.go

@@ -20,6 +20,7 @@ type InputMode int
 const (
 	ModeRouter InputMode = iota // chat / steering input
 	ModeFilter                  // session-list filter
+	ModeIntent                  // freeform request for a workflow being started
 )
 
 // EditMode is the vim-style modality: Normal = bare-key commands, Insert = typing.
@@ -146,6 +147,8 @@ type Model struct {
 	workflows  []Workflow
 	wfIndex    int // -1 = not in workflow picker
 	wfVisible  bool
+	wfPendingID   string // workflow chosen, awaiting an intent line before StartSession
+	wfPendingName string
 	bgUpdates  int
 
 	// input