Redesign README: protocol-first structure

[George-iam]

Summary

• Remove "What Is AXME?" and "AXME Cloud" sections - readers arrive already knowing what AXME is; this repo is about the protocol spec
• Add protocol-first header with tagline, badges (Alpha, License), and nav links to axme, Docs, Conformance
• Reorder sections: What Lives Here up front, then Intent Lifecycle, Delivery Bindings, Human Task Types, ScenarioBundle, Protocol Envelope, Related Repositories
• Add structured footer: hello@axme.ai, @axme_ai, Security, License
• All existing content (state machine, tables, ScenarioBundle example, Protocol Envelope diagram, Schema Governance details) preserved and unchanged

Test plan

• Verify badges render correctly on GitHub
• Verify all links resolve (nav links, footer links, diagram images)
• Verify <details> Schema Governance block expands correctly