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_collection.py

@@ -82,3 +82,15 @@ def test_query_env_var_silences_warning(col, monkeypatch, recwarn):
     col._backend.query.return_value = "answer"
     col.query("what?")
     assert not any(issubclass(w.category, UserWarning) for w in recwarn)
+
+
+def test_query_accepts_str_doc_id(col):
+    """str gets normalized to [str] internally."""
+    col._backend.query.return_value = "answer"
+    col.query("what?", doc_ids="d1")
+    col._backend.query.assert_called_once_with("papers", "what?", ["d1"])
+
+
+def test_query_rejects_empty_list(col):
+    with pytest.raises(ValueError, match="cannot be empty"):
+        col.query("what?", doc_ids=[])