Add exit code logging to macOS screencapture manager
Log sck-cli exit codes to help debug health monitoring issues:
- is_running() logs once when process exit is detected
- stop() logs exit code after SIGTERM or SIGKILL
This helps diagnose whether sck-cli exits after early video stream
failures or hangs until the boundary timeout.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>