…gging

This addresses two issues that make root cause analysis of agent crashes difficult:

1. Reaper exit code propagation: The reaper (PID 1) now returns the child's
   exit code instead of always exiting 0. Signal termination uses the standard
   Unix convention of 128 + signal number. The CLI logs the exit status and
   uses ExitError to propagate it.

2. Goroutine panic recovery: Added defer logPanic() to raw goroutines in the
   agent to ensure panics are logged with stack traces before re-panicking.
   This helps diagnose crashes that would otherwise leave no trace.

Fixes #21661