Assign readable ticket keys from project code and sequence.

Replace UUID-looking sync labels in TUI and web with keys like TAS-1,
while keeping the remote UUID for task-master matching.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/ticket/displayid_test.go

@@ -0,0 +1,15 @@
+package ticket
+
+import "testing"
+
+func TestDisplayID(t *testing.T) {
+	if got := (Ticket{Key: "TAS-1", ExternalID: "uuid"}).DisplayID(); got != "TAS-1" {
+		t.Fatalf("got %q", got)
+	}
+	if got := (Ticket{ExternalID: "YT-9"}).DisplayID(); got != "YT-9" {
+		t.Fatalf("got %q", got)
+	}
+	if got := (Ticket{}).DisplayID(); got != "" {
+		t.Fatalf("got %q", got)
+	}
+}