feat(loop): upgrade governed agent loop to the proven 6-receipt chain (retrieve->quarantine_untrusted->tool_call->policy_check->kernel_check->emit) with P3 poisoned-input non-interference; receipts signed by the persistent cosign key. Surgical: module-only (serve.py already wires + orders correctly). Signed-off-by: Stephen P. Lutar Jr. <stephenlutar2@gmail.com>