refactor(fusion_tasks): strip all sync references from task model

Remove cross-instance sync fields (x_fc_sync_source, x_fc_sync_remote_id,
x_fc_sync_uuid, x_fc_is_shadow, x_fc_sync_client_name, x_fc_sync_client_phone,
x_fc_source_label), sync push calls in create/write/action methods, shadow
task logic in constraints and email guards, and x_fc_tech_sync_id from
res.users. Also remove task_sync import from models/__init__.py.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

fusion-plating/fusion_tasks/models/__init__.py

@@ -8,6 +8,5 @@ from . import res_company
 from . import res_users
 from . import res_config_settings
 from . import technician_task
-from . import task_sync
 from . import technician_location
 from . import push_subscription