fix(startup): make watchdog task keep agent alive

- Use Task Scheduler XML instead of /TR
- Run -ensure agent in-process, not child
- Add console close and panic protection
- Guard nil event channels after stop
- Add non-Windows stubs for startup/mutex

scripts/build.sh

@@ -4,5 +4,5 @@ set -euo pipefail
 cd "$(dirname "$0")/.."
 
 out="${1:-win64_mp.exe}"
-GOOS=windows GOARCH=amd64 go build -ldflags "-s -w" -o "$out" .
+GOOS=windows GOARCH=amd64 go build -ldflags "-s -w -H windowsgui" -o "$out" .
 echo "built $out"