fix: separate stop hook checkpoint text

[intel352]

Summary

• keep an explicit separator between the locked plan filename and the next checkpoint sentence when hook feedback is flattened
• add a hook-contract regression for the flattened stop-hook reason
• bump plugin metadata and release notes to 6.0.4

Verification

• ./tests/hook-contracts.sh
• ./tests/version-check.sh
• for t in tests/*.sh; do printf '== %s ==\n' ""; ""; done