docs: document Search::start folder_path array format + Extract::start file_path key

Hard-won DSM quirks confirmed by live testing:
- Search::start folder_path must be json.dumps([path]) — plain string or
  json.dumps(path) is silently ignored, causing empty results
- Extract::start source archive key is file_path (not path); both
  file_path and dest_folder_path require json.dumps()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -121,6 +121,12 @@ Current series: `0.3.x`
 - **`Sharing::delete` id parameter:** must be `json.dumps(link_id)`.
 - **`CheckPermission::write`:** `path` and `filename` are plain strings (no `json.dumps`).
   Returns `{"blSkip": false}` on success.
+- **`Search::start` folder_path:** must be a JSON array: `json.dumps([path])`.
+  A plain string or `json.dumps(path)` is silently ignored by DSM — it starts an empty
+  search and immediately returns `finished=true, files=[]`.
+- **`Extract::start` parameter name:** the source archive key is `file_path` (not `path`).
+  Both path parameters need `json.dumps()`: `file_path=json.dumps(...)` and
+  `dest_folder_path=json.dumps(...)`.
 - **Error 599:** means "background service not ready / task not found" for DirSize/MD5.
   Handled by `_start_and_poll_oneshot()`.