agent start: a signal to the launcher should stop the released agent #668

Follow-up from #648 (sandboxed launch).

After tps agent start --sandbox-required releases the agent, a SIGTERM sent only to the launcher process removes the private config snapshot and re-raises the signal on the launcher (packages/cli/src/commands/agent.ts ~1100-1104), but nothing signals the nono supervisor or the released agent. The launcher awaits the supervisor's exit (packages/cli/src/utils/launch-attestation.ts ~973) and its finally (~974) removes private launch state without terminating either process. So stopping the launcher leaves the released agent running without its launcher. (Found by reading the source; not yet measured under real nono.)

Want: a signal to the launcher is forwarded to the nono supervisor (and through it to the agent), and the launcher waits, bounded, for both to exit before it exits itself, so stopping tps agent start stops the agent. Test (real nono in the Docker attested lane): launch, wait for release and the agent's pid file, SIGTERM the launcher only, then assert the released child pid and the supervisor are gone within the bound; mutation (do not forward the signal) turns it red.

Refs #648