Add /kuvel loadbalancer register/unregister subcommands and extend repair auto-recovery

[Copilot]

/kuvel で Pod だけでなく LoadBalancer（ReplicaSet UID）の手動登録・削除ができるようにし、運用時の復旧導線を揃えました。あわせて /kuvel repair を強化し、壊れた loadbalancer マッピングを削除前に再登録で自己修復できるようにしています。

• Command surface expansion (/kuvel)

  • Added register loadbalancer subcommand:
    • /kuvel register loadbalancer <replicaSetUid> <serverName>
  • Added unregister loadbalancer subcommand:
    • /kuvel unregister loadbalancer <replicaSetUid>
  • Kept existing pod-oriented subcommands backward compatible (register <podUid> ..., unregister <podUid>).

• Service-layer support for manual loadbalancer registration

  • Added KuvelServiceHandler#registerLoadBalancer(String replicaSetUid, String serverName).
  • Added KuvelServiceHandler#isLoadBalancerRegistered(String replicaSetUid).
  • Registration flow resolves ReplicaSet by UID under configured selectors, creates the Velocity server entry, and wires LoadBalancer metadata (strategy + initial-server flag).

• /kuvel repair auto-fix enhancement

  • For missing loadbalancer registrations, repair now attempts re-registration first.
  • Only falls back to cleanup (unregisterLoadBalancer) when re-registration fails.
  • Repair summary now reports repaired loadbalancers separately from cleaned loadbalancers.

• Docs update

  • Updated command reference in README.md and README-JP.md to include new loadbalancer subcommands and expanded repair behavior.

/kuvel register loadbalancer <replicaSetUid> <serverName>
/kuvel unregister loadbalancer <replicaSetUid>