Add new GitHub API fields

Overview

As a placeholder, until made configurable and properly documented, add the new GitHub importer API fields, further described here.

Use default values for now:

"optional_stages": {
    "single_endpoint_notes_import": False,
    "attachments_import": False,
    "collaborators_import": False
},
"timeout_strategy": "pessimistic"

Tasks

• Update docs if needed (workflow, runbooks, README, migration metrics, etc.)

Acceptance Criteria

• Existing UTs pass locally