Function replaceOrAppendMarkedSection

src/installer/targets/shared.ts:142–174

(
  filePath: string,
  body: string,
  startMarker: string,
  endMarker: string,
)

 * existing block already matches `body`.
 */
export function replaceOrAppendMarkedSection(
  filePath: string,
  body: string,
  startMarker: string,
  endMarker: string,
): 'created' | 'updated' | 'appended' | 'unchanged' {
  if (!fs.existsSync(filePath)) {
    atomicWriteFileSync(filePath, body + '\n');
    return 'created';
  }

  const content = fs.readFileSync(filePath, 'utf-8');
  const startIdx = content.indexOf(startMarker);
  const endIdx = content.indexOf(endMarker);

  if (startIdx !== -1 && endIdx > startIdx) {
    const existingBlock = content.substring(startIdx, endIdx + endMarker.length);
    if (existingBlock === body) {
      return 'unchanged';
    }
    const before = content.substring(0, startIdx);
    const after = content.substring(endIdx + endMarker.length);
    atomicWriteFileSync(filePath, before + body + after);
    return 'updated';
  }

  // No markers — append. Preserve existing content with a separating
  // blank line.
  const trimmed = content.trimEnd();
  const sep = trimmed.length > 0 ? '\n\n' : '';
  atomicWriteFileSync(filePath, trimmed + sep + body + '\n');
  return 'appended';
}

/**
 * Upsert the CodeGraph instructions block into an agent instructions