docs: Rewrite README to cover full CLI

[mslipper]

Summary

• Replaced the old quick start (login + create VM) with curl | bash then irons onboard
• Added sections for agents, VMs, secrets/env vars, egress control, snapshots, scp, port forwarding, fork, and public keys
• Removed the "schedule a call" early access section
• Moved alternate install methods (binary downloads, go install) to a smaller section near the bottom