docs: add managed runtime quickstart

[XnLemon]

Summary

• add English and Chinese Quickstarts for Runtime A registration and Runtime B -> Router -> Runtime A invocation
• include complete package main programs with imports, service startup, Nacos lease maintenance, Router credential validation, and graceful deregistration
• link the exact B handler where verified Router context is passed to agentsdk.Client.Invoke
• provide a copy/paste NeKiro-Stack backend acceptance path with secured Nacos and Ledger assertions
• separate product E2E from Core development checks and document the trust/ownership boundaries

Verification

• Go syntax/gofmt parsing for all four README program blocks
• English and Chinese program blocks are byte-identical
• go build ./...
• go vet ./...
• gofmt -l apps config_center contracts tests
• git diff --check
• prior revision: go test ./... (one transient Windows file rename denial in config_center/file; the failed test and full package passed on immediate rerun)

Fallback delta: removed 0, retained 0, added 0, net +0
Added fallback evidence: none

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!