fix(mac): resolve duplicate symbols blocking darwin/linux builds + add Mac tests. Remove duplicate launchProcessGuard from guard_unix.go (kept richer health_unix.go shell-loop version using pgrep -f). Remove duplicate applyDetachedStart from exec_stub.go (kept platform_unix.go impl that clears stdio). Add silent_stub.go for !windows so silentCombinedOutput/silentOutput/silentRun compile on Mac. All 4 targets now build: darwin/arm64, darwin/amd64, linux/amd64, linux/arm64. New tests: workerProcessRunning, applyDetachedStart stdio clearing, PersistenceKeyName, HostOSVersion, LaunchAgent plist content, silent stub exec wrappers.

agent/deploy/exec_stub.go

@@ -26,4 +26,3 @@ func HiddenOutput(name string, arg ...string) ([]byte, error) {
 	return exec.Command(name, arg...).Output()
 }
 
-func applyDetachedStart(cmd *exec.Cmd) {}