[SRE-5664] Phase 5o: Add SSH agent support to builder image

[itskingori]

Adds OpenSSH client tools to the builder image so workloads can use SSH agent forwarding during build steps. The Phase 5o port-control-app builder implementation needs to support SSH-agent-backed authentication, while keeping this image client-only and avoiding an SSH server.

This image change provides ssh-agent and ssh-add through Ubuntu's openssh-client package for the follow-up workflow introduced there.

Changes

• Installs openssh-client in the builder image
• Updates the documented image contents to include OpenSSH client tools
• Adds a 1.2.0 changelog entry for SSH agent support

Testing

• GitHub Actions build workflow passes for this branch.

Related

• https://github.com/Intellection/port-control-app/pull/126