docs: add protocol concepts to README

[George-iam]

Summary

• Rewrote README.md with expanded protocol concepts: intent lifecycle diagram, delivery bindings table, runtime steps (internal steps + human task types), and ScenarioBundle usage example
• Moved schema governance sections (versioning, compatibility, payload extensibility, error model, integration rule, validation) into a collapsible details block for contributors
• Replaced all em dashes with regular dashes for consistency
• Preserved all SVG diagram links, Related Repositories table, and directory tree unchanged

Test plan

• Verify all SVG diagram links render correctly on GitHub
• Confirm markdown tables render properly
• Check collapsible details section expands/collapses as expected