Python: agent-hooks tool-seam halt keys on the host_error: prefix, catching composition-produced denies

The agent-hooks feature halts the run when a tool-seam deny carries a `host_error:*` reason (enforcement-layer failure — correct). But the check keys on the reason prefix, so it also fires for composition-*produced* denies like `host_error:transform_conflict` and `host_error:composition_disagreement`, which are policy outcomes of the configured profile rather than enforcement-layer failures — arguably those should follow the ordinary tool-deny path (reason-only error to the model, loop continues). Worth deciding deliberately either way; found during conformance cross-validation (responsibleai/agent-hooks conformance/claims/maf/REPORT.md).