feat: add fetched-at timestamp to markdown header and json meta

tests/render_markdown_tests.rs

@@ -12,6 +12,7 @@ fn render_markdown_includes_expected_sections_and_preserves_comment_markdown() {
             pr_index: 7,
             title: "Fix parser".into(),
             description: Some("PR body".into()),
+            fetched_at: Some("2026-04-08T12:34:56Z".into()),
             state: "open".into(),
             author: "alice".into(),
             base_branch: "main".into(),
@@ -72,6 +73,7 @@ fn render_markdown_includes_expected_sections_and_preserves_comment_markdown() {
 
     assert!(md.contains("# org/repo `#7` Fix parser"));
     assert!(md.contains("## Metadata"));
+    assert!(md.contains("> fetched at: 2026-04-08T12:34:56Z"));
     assert!(md.contains("## Commits"));
     assert!(md.contains("## Diff Stat"));
     assert!(md.contains("> 编号规则：Review `<pr>.<review>`；Comment `<pr>.<review>.<comment>`；Reply `<pr>.<review>.<comment>.<reply>`"));
@@ -99,6 +101,7 @@ fn render_markdown_uses_minimal_fence_for_plain_text() {
             pr_index: 1,
             title: "T".into(),
             description: None,
+            fetched_at: None,
             state: "open".into(),
             author: "alice".into(),
             base_branch: "main".into(),