feat(cli): add bounded graceful server shutdown

Priority

Wave 3 — user-facing lifecycle control. Implement after #3160 establishes durable local ownership and a bounded shutdown API.

Problem

Users need a supported way to stop Gym services without tracking terminals or killing individual PIDs. The command must consume #3160 rather than create a second process-discovery and termination path.

Essential scope

• Stop all locally owned services or a selected service.
• Use #3160's one graceful deadline and explicit force escalation.
• Report graceful, timed-out, force-terminated, already-stopped, and unresolved-remote-cleanup results.
• Make repeated stop safe.
• Document graceful versus forced local shutdown and the limits of remote cleanup.

Dependencies

Hard dependency on #3160's durable process ownership and shutdown API. Remote release state is consumed when #2609/#3037 provides it but does not block the local command.

Acceptance criteria

• Users can stop all services or one selected service through gym env.
• Graceful shutdown uses one overall deadline.
• Force escalation is explicit and never the first default action.
• Repeating stop is safe when services are already stopped.
• Local exit and unresolved remote cleanup are reported separately.