F3 SLICE-5 review fixes: fence-aware replacement extraction + editSelection guards (#6)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/liveTurn.ts

@@ -24,8 +24,15 @@ const SYSTEM_PROMPT = [
   "unless the instruction says otherwise.",
 ].join(" ");
 
-/** Strip a wrapping markdown code fence if the model added one anyway. */
-export function extractReplacement(outputText: string): string {
+/**
+ * Strip a wrapping markdown code fence if the model added one anyway.
+ * If `selectedText` itself was fence-wrapped (i.e. the user's selection was a
+ * fenced block), the model mirroring that format is correct — return
+ * `outputText` unchanged so the fence is preserved.
+ */
+export function extractReplacement(outputText: string, selectedText: string): string {
+  const fencePattern = /^\s*```[^\n]*\n[\s\S]*?\n?```\s*$/;
+  if (fencePattern.test(selectedText)) return outputText;
   const m = outputText.match(/^\s*```[^\n]*\n([\s\S]*?)\n?```\s*$/);
   return m ? m[1] : outputText;
 }
@@ -53,5 +60,5 @@ export async function runEditTurn(
         "(is Claude Code installed and signed in?)",
     );
   }
-  return { replacement: extractReplacement(result.outputText), model: modelId, sessionId: result.runId };
+  return { replacement: extractReplacement(result.outputText, selectedText), model: modelId, sessionId: result.runId };
 }