Add Subtasks association to the findTaskByIdentifier function
so that the get_task MCP API endpoint returns subtasks along
with the main task. This enables clients to access the full
task hierarchy in a single API call.
The serializeTask function already supported subtasks
serialization, so this change only required updating the
query includes to load the Subtasks relation with proper
ordering and Tag associations.
Fixes#1029
46329fc82b