feat: replace MCP attachment tool with proxy endpoint

The MCP tool approach was impractical because it required the LLM to
generate large base64 strings token-by-token, causing timeouts.

Changes:
- Remove upload_attachment MCP tool
- Add POST /api/v1/attachments endpoint for multipart/form-data uploads
- Update proxy documentation to show both endpoints
- Uses existing GristClient.upload_attachment() method
- Requires write permission in session token

tests/unit/test_tools_session.py

@@ -39,12 +39,14 @@ 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 "endpoints" in result
+    assert "proxy" in result["endpoints"]
+    assert "attachments" in result["endpoints"]
     assert "authentication" in result
     assert "methods" in result
     assert "add_records" in result["methods"]
     assert "get_records" in result["methods"]
+    assert "attachment_upload" in result
     assert "example_script" in result