Implement atomic file operations for URL fetch caching

[Copilot]

URL content caching in runtime_import.cjs was vulnerable to race conditions and partial writes when multiple workflow runs fetched the same URL concurrently.

Changes

• Atomic write-then-rename pattern in fetchUrlContent():
  • Write to temporary file: ${cacheFile}.tmp.${process.pid}.${Date.now()}
  • Atomic rename via fs.renameSync() (POSIX guarantee)
  • Cleanup temp file on error

// Write to temporary file first
const tempFile = `${cacheFile}.tmp.${process.pid}.${Date.now()}`;
fs.writeFileSync(tempFile, data, "utf8");

// Atomic rename - file is either complete or absent
fs.renameSync(tempFile, cacheFile);

This ensures cache files are never partially written - concurrent readers either see the complete file or no file at all.

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