Open instance ports from the CLI

[callen-bot]

Summary

• add brev ports open <instance-or-node> <port> with add as an alias
• support TCP, UDP, and SSH protocols plus repeatable --allow <CIDR> source restrictions
• print the allocated mapping as a table or stable JSON and document the workflow in the Brev CLI skill

Why

PR #441 exposes existing port mappings in the CLI, but users still have to switch to the console to open one. This stacked follow-up mirrors the console raw-port flow for managed instances and registered compute nodes.

Impact

Users can expose a destination port without leaving the terminal, choose the protocol, restrict inbound sources, and consume the allocated public mapping in scripts.

Stack

• Base: Add ports command for instances and nodes #441
• This PR contains only the port-opening follow-up.

Validation

• go test -race ./pkg/cmd/ports ./pkg/cmd/util ./pkg/cmd
• make fmtcheck
• make fast-build
• make vet
• make lint