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

openclaw.plugin.json

@@ -139,10 +139,9 @@
           },
           "max_results": {
             "type": "integer",
-            "description": "Maximum number of chunks to return",
-            "default": 5,
+            "description": "Maximum number of chunks to return (default: unlimited)",
             "minimum": 1,
-            "maximum": 50
+            "maximum": 10000
           },
           "directory_filter": {
             "type": "array",