feat(collection): doc_ids accepts str|list, design cleanups

- Collection.query and Backend.query/query_stream accept doc_ids as
  str, list[str] or None. Single str is normalized to [str] inside each
  backend; bare [] is rejected with ValueError at both layers.
- wrap_with_doc_context wraps the scoped doc list in <docs>...</docs>
  and SCOPED_SYSTEM_PROMPT instructs the agent to treat that block as
  data, not instructions (defense against prompt injection via
  auto-generated doc_description).
- _require_cloud_api now distinguishes api_key="" from api_key=None;
  the former gives a targeted error pointing at the empty-string vs
  fall-back-to-local situation when legacy SDK methods are called.
- Legacy PageIndexClient.list_documents docstring spells out the
  return-shape difference vs collection.list_documents() to flag a
  silent migration footgun (paginated dict with id/name keys vs plain
  list[dict] with doc_id/doc_name keys).
- Remove dead CloudBackend.get_agent_tools stub (not on the Backend
  protocol; only ever returned an empty AgentTools()) and the
  SYSTEM_PROMPT alias (OPEN_/SCOPED_SYSTEM_PROMPT are the explicit
  names now).
- README quick start and streaming example now pass doc_ids; new
  multi-document section shows both str and list forms.
- examples/demo_query_modes.py exercises all five query-mode cases
  (single-doc, multi-doc with/without env var, scoped single, scoped
  multi) for manual verification.

tests/test_cloud_backend.py

@@ -1,3 +1,5 @@
+import pytest
+
 from pageindex.backend.cloud import CloudBackend, API_BASE
 
 
@@ -11,6 +13,7 @@ def test_api_base_url():
     assert "pageindex.ai" in API_BASE
 
 
-def test_get_retrieve_model_is_none():
+def test_query_rejects_empty_doc_ids():
     backend = CloudBackend(api_key="pi-test")
-    assert backend.get_agent_tools("col").function_tools == []
+    with pytest.raises(ValueError, match="cannot be empty"):
+        backend.query("col", "q", doc_ids=[])