Rewrite the README for a public audience

[ZILECAO]

Summary

• rewrite the README for a general open-source audience
• lead with the simple product outcome: private, persistent EC2 workspaces for coding agents
• remove company- and employee-specific language while preserving the real Identity Center access model
• remove the broken workflow badge, which cannot read this private repository
• keep the useful license, security, architecture, setup, operations, and cost information

Validation

• ./scripts/check.sh
• GitHub Markdown API render of README.md
• verified every local README link
• verified both remaining Shields badge targets
• xmllint --noout assets/agentformation-demo.svg

This changes documentation only. It does not alter authentication, AWS resources, or runtime behavior.