feat(plugin): add ReMe integration for OpenClaw

**Background**

OpenClaw users need a native way to use ReMe as persistent, file-native memory across agent sessions. The integration should follow OpenClaw plugin conventions while keeping user-owned ReMe workspace files as the source of truth and connecting to a separately managed ReMe service.

**Changes**
- Add an installable OpenClaw plugin under plugins/openclaw
- Expose ReMe recall workflows through OpenClaw-compatible tools and skills
- Use session lifecycle hooks to record durable session knowledge automatically without blocking agent shutdown
- Support semantic search, graph traversal, state lookup, and source-backed reads
- Add configuration for the ReMe service endpoint and clear health and failure handling
- Add installation documentation and focused tests for recall and automatic recording