feat(at): add ssh and env:set commands to ac-at CLI
- `ac-at ssh` — interactive SSH into PDS droplet
- `ac-at ssh <command>` — run remote command on PDS
- `ac-at env:set <KEY> <VALUE>` — set env var in /pds/pds.env
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>