Under a multiplexed gateway every profile's outbound webhooks share one
delivery worker, so receivers could not tell which profile fired an
event. Add a top-level `profile` field to the payload, resolved at fire
time from the bound Hermes home via get_active_profile_name() ("default"
outside profiles). Documents the field in the wire-format section.
Reported by @vszgdcn8cj-ctrl.
Fixes #92674