feat: complete OpenClaw plugin migration with SQLite persistence
- Migrate to OpenClaw Plugin SDK with definePluginEntry and api.registerTool
- Rewrite GraphDatabase with better-sqlite3 for persistent storage
- Implement recall depth traversal, timeRange filtering, supersede mode
- Add archive and cleanup data management features
- Fix SKILL.md frontmatter to OpenClaw single-line format (kebab-case names)
- Add ToolLimiter for per-turn rate limiting
- Add TypeBox parameter schemas for OpenClaw tool registration
- Add 110 tests across 4 test files (GraphDatabase, MemoryService, Tool, Limiter)
- Update README for OpenClaw plugin installation and usage
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