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.

src/grist_mcp/tools/session.py

@@ -0,0 +1,106 @@
+"""Session token tools for HTTP proxy access."""
+
+PROXY_DOCUMENTATION = {
+    "description": "HTTP proxy API for bulk data operations. Use request_session_token to get a short-lived token, then call the proxy endpoint directly from scripts.",
+    "endpoint": "POST /api/v1/proxy",
+    "authentication": "Bearer token in Authorization header",
+    "request_format": {
+        "method": "Operation name (required)",
+        "table": "Table name (required for most operations)",
+    },
+    "methods": {
+        "get_records": {
+            "description": "Fetch records from a table",
+            "fields": {
+                "table": "string",
+                "filter": "object (optional)",
+                "sort": "string (optional)",
+                "limit": "integer (optional)",
+            },
+        },
+        "sql_query": {
+            "description": "Run a read-only SQL query",
+            "fields": {"query": "string"},
+        },
+        "list_tables": {
+            "description": "List all tables in the document",
+            "fields": {},
+        },
+        "describe_table": {
+            "description": "Get column information for a table",
+            "fields": {"table": "string"},
+        },
+        "add_records": {
+            "description": "Add records to a table",
+            "fields": {"table": "string", "records": "array of objects"},
+        },
+        "update_records": {
+            "description": "Update existing records",
+            "fields": {"table": "string", "records": "array of {id, fields}"},
+        },
+        "delete_records": {
+            "description": "Delete records by ID",
+            "fields": {"table": "string", "record_ids": "array of integers"},
+        },
+        "create_table": {
+            "description": "Create a new table",
+            "fields": {"table_id": "string", "columns": "array of {id, type}"},
+        },
+        "add_column": {
+            "description": "Add a column to a table",
+            "fields": {
+                "table": "string",
+                "column_id": "string",
+                "column_type": "string",
+                "formula": "string (optional)",
+            },
+        },
+        "modify_column": {
+            "description": "Modify a column's type or formula",
+            "fields": {
+                "table": "string",
+                "column_id": "string",
+                "type": "string (optional)",
+                "formula": "string (optional)",
+            },
+        },
+        "delete_column": {
+            "description": "Delete a column",
+            "fields": {"table": "string", "column_id": "string"},
+        },
+    },
+    "response_format": {
+        "success": {"success": True, "data": "..."},
+        "error": {"success": False, "error": "message", "code": "ERROR_CODE"},
+    },
+    "error_codes": [
+        "UNAUTHORIZED",
+        "INVALID_TOKEN",
+        "TOKEN_EXPIRED",
+        "INVALID_REQUEST",
+        "GRIST_ERROR",
+    ],
+    "example_script": """#!/usr/bin/env python3
+import requests
+import sys
+
+token = sys.argv[1]
+host = sys.argv[2]
+
+response = requests.post(
+    f'{host}/api/v1/proxy',
+    headers={'Authorization': f'Bearer {token}'},
+    json={
+        'method': 'add_records',
+        'table': 'Orders',
+        'records': [{'item': 'Widget', 'qty': 100}]
+    }
+)
+print(response.json())
+""",
+}
+
+
+async def get_proxy_documentation() -> dict:
+    """Return complete documentation for the HTTP proxy API."""
+    return PROXY_DOCUMENTATION

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