Document Space Chain APIs in README

[mashenjun]

Problem

README.md does not document the new Space Chain API surface.

Scope

Update the API Reference to include:

1. The /v1alpha2/space-chains* management endpoints.
2. chain_ key behavior and authentication expectations.
3. Runtime support for chain_ keys on /v1alpha2/mem9s/* memory, import, and session-message APIs.
4. Relevant request/response examples for creating chains, replacing nodes, managing bindings, and using a chain key at runtime.

Context

The implemented routes are wired in server/internal/handler/handler.go, but README.md currently only lists the standard /v1alpha2/mem9s/memories endpoints.