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/res_users.py

@@ -18,9 +18,3 @@ class ResUsers(models.Model):
         readonly=False,
         string='Start Location',
     )
-    x_fc_tech_sync_id = fields.Char(
-        string='Tech Sync ID',
-        help='Shared identifier for this technician across Odoo instances. '
-             'Must be the same value on all instances for the same person.',
-        copy=False,
-    )