refactor(runtime): use the SDK host for A/B lifecycle

[XnLemon]

Problem

Runtime A and Runtime B duplicate HTTP serving, registration lease observation, cancellation, bounded shutdown, and deregistration. This makes the sample entrypoints long and allows lifecycle behavior to drift.

Proposal

After NeKiro-project/nekiro-sdk-go#3 is accepted, migrate both entrypoints to agent/host while retaining their runtime-owned configuration, Nacos TLS/mTLS transport, Router authentication, endpoint challenge, and Handler construction.

Acceptance criteria

• Runtime A and B use the same host lifecycle implementation.
• Direct registration-disabled mode remains explicit and supported.
• Registration, serve, heartbeat, shutdown, and deregistration failures preserve staged errors.Is/As behavior.
• Existing unit and E2E behavior remains unchanged.
• The SDK dependency is pinned to one immutable revision.

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

[XnLemon]

Implemented in Draft PR #12, depending on SDK PR #4.

• Runtime A and Runtime B now use the shared agent/host lifecycle.
• Runtime-owned configuration, Router auth, Nacos TLS/mTLS, endpoint challenge, and handlers remain unchanged.
• Direct registration-disabled mode remains explicit.
• Added cmd-level tests for staged configuration failures and removed duplicated lifecycle code.

Validation: go test ./..., go test -race ./..., go vet ./...; Runtime A/B, security, images, and required CI are green.

[XnLemon]

Completed by #12 and merged as ce988dc16e536e02e4ea94848f1362cb6acc071e. Runtime A/B now use the SDK host while retaining Samples-owned configuration, security, readiness, and Agent behavior.

中文

已由 #12 完成，并以 ce988dc16e536e02e4ea94848f1362cb6acc071e 合并。Runtime A/B 现已使用 SDK host，同时继续由 Samples 拥有配置、安全、readiness 和 Agent 行为。