docs(readme): the okf server as a lookup during a run, with a validated example config

README section with a before/after command and the limits stated plainly (link not yet tried in a
real run, agent adherence not measured, --prepass-payload rejects okf-consumption/2). Adds
examples/okf-server.mcp.json, pinned by a test that loads it through load_mcp_config; the
published-surface pin moves 516 -> 518 for the two new files.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_okf_server_example.py

@@ -0,0 +1,24 @@
+"""The committed example that wires the ``okf`` MCP server into a run must load through the same
+fail-fast loader a real run uses -- a config in the README that ``load_mcp_config`` refuses is a
+broken instruction, not documentation."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from portfolio_optimiser.mcp_tools import load_mcp_config
+
+EXAMPLE = Path(__file__).resolve().parent.parent / "examples" / "okf-server.mcp.json"
+
+
+def test_okf_server_example_loads_through_the_run_loader() -> None:
+    (server,) = load_mcp_config(EXAMPLE)
+    assert server.name == "okf"
+    assert server.transport == "stdio"
+    assert set(server.allowed_tools) == {"okf_list", "okf_describe", "okf_ask", "okf_fetch"}
+    assert server.timeout_seconds > 0
+
+
+def test_okf_server_example_carries_a_neutral_placeholder_path() -> None:
+    text = EXAMPLE.read_text(encoding="utf-8")
+    assert "/Users/" not in text and "/home/" not in text