- Add utils.sh with cross-platform timeout functions for macOS compatibility
- Fix sf-logs-tail to use portable_timeout instead of GNU timeout command
- Fix test-all-wrappers.sh to use portable_timeout_seconds for testing
- Update README.md with macOS compatibility documentation
The timeout command is not available by default on macOS, causing
sf-logs-tail and test scripts to fail. The new utils.sh provides
fallback timeout functionality that works on Linux, macOS with/without
GNU coreutils, maintaining exit code 124 compatibility.
