docs: add MCP quick-start as primary integration path

feat: add MCP Server for plug-and-play agent integration

fix: update payment_service to self-facilitated x402, fix GetPaymentInfo check

Add x402 client + complete agent tutorial

Use clawmesh.duckdns.org domain in all examples

Update README with comprehensive docs, fix spec links

Initial release: SAP-8183, SAP-8004, Skills API