feat: add transcript parser for last user/agent messages

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

internal/transcript/transcript_test.go

@@ -0,0 +1,21 @@
+package transcript
+
+import "testing"
+
+func TestLastMessages(t *testing.T) {
+	u, a, err := LastMessages("testdata/sample.jsonl", 2000)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if u != "second question" || a != "final answer" {
+		t.Fatalf("got user=%q agent=%q", u, a)
+	}
+}
+
+func TestTruncate2000(t *testing.T) {
+	u, a, err := LastMessages("testdata/long.jsonl", 2000)
+	if err != nil || len(a) != 2000 {
+		t.Fatalf("len=%d err=%v", len(a), err)
+	}
+	_ = u
+}