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_server.py

@@ -52,14 +52,13 @@ tokens:
     assert "add_column" in tool_names
     assert "modify_column" in tool_names
     assert "delete_column" in tool_names
-    assert "upload_attachment" in tool_names
 
     # Session tools (always registered)
     assert "get_proxy_documentation" in tool_names
     assert "request_session_token" in tool_names
 
-    # Should have all 15 tools
-    assert len(result.root.tools) == 15
+    # Should have all 14 tools
+    assert len(result.root.tools) == 14
 
 
 @pytest.mark.asyncio