fix(flows): expose unrecoverable turn failures

[DongyunZou]

Summary

• expose Unrecoverable through the backend-neutral hmz.flows API
• document why flow authors need the exception in bounded retry loops
• add a contract test proving the flow-facing name is the agent-layer exception object
• remove the pre-existing README trailing whitespace that made the repository's all-files pre-commit CI fail on a clean runner

Why

The official humanize1:gen-plan flow must distinguish transient turn failures from failures that cannot succeed on retry. Importing hmz.agents directly would violate the flow layering contract, so the vocabulary needs to be forwarded by hmz.flows.

This is the prerequisite for humanfia/flowverse#2, which contains the complete backend-neutral gen-plan convergence fix.

Verification

• uv run pytest -q tests/test_flow_interface.py
• uv run pre-commit run --all-files
• GitHub CI check job passed on commit d66ad04