fix: add configSchema to openclaw.plugin.json, add search CLI command, fix total_docs stat
- Add required configSchema to openclaw.plugin.json for OpenClaw plugin discovery
- Add search command to CLI with --limit, --dir, --from-date, --to-date, --tags filters
- Fix get_stats() to properly count unique docs (was returning 0 for non-null values)
- Remove hardcoded max_results default of 5; search now returns all results by default
- Update INSTALL.md and design docs with correct OpenClaw extension path instructions