fix: suppress hook locale warnings

[intel352]

Summary

• normalize unavailable C.UTF-8 locale values in the shared hook wrapper before launching hook scripts
• add a wrapper-level hook contract regression proving valid JSON with no stderr under C.UTF-8 env vars
• bump plugin metadata and release notes to 6.0.3

Verification

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

Note: local direct-hook tests still print ambient shell locale warnings before the wrapper is involved; the new regression captures the actual plugin wrapper path and asserts its stderr is empty.