feat(collection): scoped query mode and experimental multi-doc warning

- get_agent_tools branches on doc_ids:
  - scoped (doc_ids=[...]): drops list_documents and hard-enforces a
    whitelist on the remaining tools; system prompt switches to
    SCOPED_SYSTEM_PROMPT (no list_documents instruction); doc list +
    summaries are prepended to the user message via wrap_with_doc_context.
  - open (doc_ids=None): unchanged 4-tool agent loop.
- list_documents now exposes doc_description (sqlite + cloud).
- Collection.query emits UserWarning when doc_ids is None and the
  collection holds >1 documents; PAGEINDEX_EXPERIMENTAL_MULTIDOC=1
  silences it. Single-doc collections skip the warning; empty
  collections raise ValueError.
- Agents SDK tracing upload disabled by default (avoids SSL timeouts);
  PAGEINDEX_AGENTS_TRACING=1 re-enables it.
- README: new SDK Usage section covering local/cloud quick start,
  streaming, multi-doc as experimental, and runnable examples.

pageindex/backend/cloud.py

@@ -216,7 +216,12 @@ class CloudBackend:
             params["folder_id"] = folder_id
         data = self._request("GET", "/docs/", params=params)
         return [
-            {"doc_id": d.get("id", ""), "doc_name": d.get("name", ""), "doc_type": "pdf"}
+            {
+                "doc_id": d.get("id", ""),
+                "doc_name": d.get("name", ""),
+                "doc_description": d.get("description", ""),
+                "doc_type": "pdf",
+            }
             for d in data.get("documents", [])
         ]