docs: define layered errors and streamline quickstart

[XnLemon]

Summary

• document layered error ownership across Core, contracts, and the SDK Runtime host
• move the runnable Stack acceptance command to the top of both Quickstarts
• keep complete, SDK-host-based Runtime A/B package main examples in collapsible sections
• make English and Chinese code blocks byte-identical to the Samples Runtime mains

Error design

Core keeps owner-local typed errors in config_center, registry, and the Control Plane/Router domains. contracts.PlatformErrorCode remains the stable wire/API layer. Boundary adapters map errors explicitly and redact dependency details. nekiro-sdk-go/agent/host owns Runtime lifecycle stages; Core does not gain a global errors package or catch layer.

Quickstart flow

The README now leads with the real Stack backend acceptance and its success signals, then explains:

Runtime A -> Nacos lease -> Router snapshot/watch -> Runtime B -> Router -> Runtime A -> Ledger

The full sources remain linked to NeKiro-Samples, including nested B -> A invocation.

Dependencies

• feat(agent): add staged Runtime host nekiro-sdk-go#4
• refactor(runtime): use the SDK host lifecycle NeKiro-Samples#12

Closes #116

Verification

• gofmt -l apps config_center contracts tests
• git diff --check
• go build ./...
• go test ./...
• go test -race ./...
• go vet ./...

Fallback delta

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!

[XnLemon]

LGTM. I rechecked current HEAD 134f37c after SDK #4 and Samples #12 merged. The one required review closure was fixed in 134f37c: ADR 0020 is now Accepted for issue #116, matching the completed decision and repository ADR convention. No other blocking findings remain. The Quickstart leads with the executable Stack path, the English and Chinese Go blocks are byte-identical and match the merged Samples mains, and the layered error ownership does not introduce a global taxonomy or fallback. Validation: git diff --check, Go snippet gofmt, go build ./..., go test ./..., go test -race ./..., go vet ./..., and every required CI check passed.

中文

LGTM。SDK #4 与 Samples #12 合并后，我重新核对了当前 HEAD 134f37c。唯一需要闭合的 review 问题已在 134f37c 修复：ADR 0020 现为 Accepted for issue #116，与已完成的决策及仓库 ADR 约定一致。没有其他阻断项。Quickstart 先展示可执行 Stack 链路；中英文 Go 代码逐字一致，并与已合并的 Samples main 对齐；分层错误归属没有引入全局 taxonomy 或 fallback。验证：git diff --check、Go snippet gofmt、go build ./...、go test ./...、go test -race ./...、go vet ./... 以及全部 required CI 均已通过。