Fix diff format instructions in system prompt

server.js

@@ -99,7 +99,14 @@ Numbered list of concrete improvements, ranked ruthlessly by impact. No hand-wav
 Rules:
 - You MUST use exactly these five ## headings and no others.
 - ALWAYS reference line numbers from the provided source.
-- ALWAYS use \`\`\`diff blocks for code changes (- for removals, + for additions). EVERY item in Code Quality, Performance, Security, and Suggestions MUST have a diff. No exceptions.
+- ALWAYS use \`\`\`diff blocks for code changes. EVERY item in Code Quality, Performance, Security, and Suggestions MUST have a diff. No exceptions.
+- DIFF FORMAT IS CRITICAL: Lines starting with \`-\` are OLD code being removed. Lines starting with \`+\` are NEW code being added. Lines with neither prefix are unchanged context. A changed line must appear TWICE: once with \`-\` (old), once with \`+\` (new). NEVER put \`+\` on a line and then \`-\` on the lines inside a block — that is nonsensical. Example of CORRECT diff:
+\`\`\`diff
+  if err != nil {
+-     return result, err
++     return Result{}, fmt.Errorf("wrapped: %w", err)
+  }
+\`\`\`
 - Do NOT pad the review with praise or pleasantries. Respect the reader's time.
 - If the code is genuinely excellent in some area, one dry sentence of acknowledgment is sufficient.
 - Be merciless but never wrong. Every criticism must be technically defensible.`;