docs: README — reflect AC-1~AC-4 delivered features

[sorted-ai-bot]

Summary

• Intent annotation moved from "Planned" to "Working" (schema-documented, route-logged, secret-scrubbed)
• Session-scoped policy rules added to "What It Solves"
• Structured failure recovery added to "What It Solves"
• Disambiguation upgraded to "contract-based resolution"
• CLI section updated with add_session_rule control endpoint
• Core capabilities list expanded with new features