feat: enforce output version v1 and add version subcommand

tests/render_json_tests.rs

@@ -5,12 +5,13 @@ use gitea_pr_review::model::{
 #[test]
 fn model_json_roundtrip() {
     let doc = PrReviewDocument {
+        version: "v1".into(),
         meta: PrMeta {
             repo: "org/repo".into(),
             pr_index: 9,
             title: "feat: x".into(),
             description: Some("desc".into()),
-            fetched_at: Some("2026-04-08T12:34:56Z".into()),
+            fetched_at: "2026-04-08T12:34:56Z".into(),
             state: "open".into(),
             author: "alice".into(),
             base_branch: "main".into(),
@@ -50,7 +51,8 @@ fn model_json_roundtrip() {
 
     let encoded = serde_json::to_string(&doc).unwrap();
     let decoded: PrReviewDocument = serde_json::from_str(&encoded).unwrap();
+    assert_eq!(decoded.version, "v1");
     assert_eq!(decoded.meta.repo, "org/repo");
-    assert_eq!(decoded.meta.fetched_at.as_deref(), Some("2026-04-08T12:34:56Z"));
+    assert_eq!(decoded.meta.fetched_at, "2026-04-08T12:34:56Z");
     assert_eq!(decoded.threads[0].root_comment.body, "hello");
 }