feat(cli): Add `nemoclaw gateway delete` command with optional `--destroy` flag

[pimlock]

Problem

Currently, removing a gateway from the local list requires using nemoclaw gateway destroy, which:

1. Doesn't accept a gateway address as an argument — you must first select the gateway interactively
2. Implies infrastructure destruction — the name "destroy" suggests the remote gateway (e.g. running on Brev) will be torn down, which isn't always the case. Sometimes you just want to remove it from your local list.

Proposed Solution

Add a nemoclaw gateway delete <address> command:

• nemoclaw gateway delete <address> — removes the gateway from the local list only (no infrastructure changes)
• nemoclaw gateway delete <address> --destroy — removes from the local list and tears down the gateway infrastructure

The --destroy flag should only work on gateways that are local or remote SSH (where we actually manage the lifecycle). For managed/hosted gateways (e.g. Brev), --destroy should error with a clear message.

Acceptance Criteria

• nemoclaw gateway delete <address> removes a gateway from the local configuration
• nemoclaw gateway delete <address> --destroy tears down the gateway and removes it from local config
• --destroy is rejected for gateway types where we don't control the lifecycle (e.g. Brev-hosted)
• Existing nemoclaw gateway destroy behavior is preserved or aliased for backward compatibility