feat: add render-md json input flow

src/render/json.rs

@@ -0,0 +1,9 @@
+use crate::model::PrReviewDocument;
+
+pub fn render_json(doc: &PrReviewDocument) -> anyhow::Result<String> {
+    Ok(serde_json::to_string_pretty(doc)?)
+}
+
+pub fn parse_json(input: &str) -> anyhow::Result<PrReviewDocument> {
+    Ok(serde_json::from_str(input)?)
+}