fix: distinguish worker crash from timeout, show actual error message

Previously both worker crash (stdout closed) and real timeout showed
the same "Worker timeout" error. Now:
- Worker crash: reads cached stderr error lines, shows actual error
  (e.g. "CUDA OOM", "segfault") with exit code
- Real timeout: shows timeout duration and suggestion to reduce resolution
- Default timeout increased to 1800s for all _call operations
- _stderr_reader caches recent error/fatal lines for crash diagnostics

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>