feat(tools): add get_proxy_documentation tool

Add a new MCP tool that returns complete documentation for the HTTP
proxy API. This enables agents to get all the information they need
to construct valid proxy requests when writing scripts.

The tool is stateless and returns a static documentation dict
describing endpoints, methods, authentication, and example usage.

tests/unit/test_tools_session.py

@@ -0,0 +1,16 @@
+import pytest
+from grist_mcp.tools.session import get_proxy_documentation
+
+
+@pytest.mark.asyncio
+async def test_get_proxy_documentation_returns_complete_spec():
+    result = await get_proxy_documentation()
+
+    assert "description" in result
+    assert "endpoint" in result
+    assert result["endpoint"] == "POST /api/v1/proxy"
+    assert "authentication" in result
+    assert "methods" in result
+    assert "add_records" in result["methods"]
+    assert "get_records" in result["methods"]
+    assert "example_script" in result