feat(cli): support custom SSH port for remote gateway deployment

[bokerh]

Problem Statement

openshell gateway add ssh://user@host:8080 parses 8080 as the gateway's HTTPS port. There is no way to specify a custom SSH port (e.g. 2222) for the connection itself. The CLI strips the SSH connection down to user@host, which forces the underlying Docker library to default to port 22.

Proposed Design

1. CLI Flag: Add --ssh-port <PORT> to remote commands (gateway add, gateway start, etc).
2. Internal API: Add ssh_port: Option<u16> to RemoteOptions.
3. Execution: In create_ssh_docker_client, append the port to the bollard connection string if set (ssh://user@host:PORT).

Alternatives Considered

• ~/.ssh/config alias: Users define a Host with a custom Port. Rejected: This is currently broken. resolve_ssh_hostname in metadata.rs only parses the hostname line from ssh -G and ignores the port line, so the custom port is lost before the connection starts.
• URI overloading: Support ssh://user@host:2222:8080. Rejected: Invalid URL format.

Agent Investigation

• crates/openshell-cli/src/run.rs L925: URL parser assigns the parsed port to the gateway.
• crates/openshell-bootstrap/src/lib.rs L59: RemoteOptions struct has no ssh_port field.
• crates/openshell-bootstrap/src/docker.rs L240: create_ssh_docker_client passes ssh://user@host to bollard without a port.
• crates/openshell-bootstrap/src/metadata.rs L164: resolve_ssh_hostname ignores the port field from ssh -G output.

Checklist

• I've reviewed existing issues and the architecture docs
• This is a design proposal, not a "please build this" request