fix: parse POSIX block-monk fallback commands

[renaissancefieldlite]

Fixes #55.

Summary

• parse the actual hook command payload in POSIX fallback mode when monk-agent is unavailable
• deny quoted, backslash-escaped, sudo, command, and simple env wrapper forms that still invoke the monk CLI
• keep harmless argument uses like grep monk README.md and grep "monk" README.md allowed
• apply the fallback fix to the root, packaged Monk, and Antigravity hook copies

Verification

• python3 tests/block-monk-posix.py
• sh -n .antigravity-plugin/hooks/block-monk.sh
• sh -n hooks/block-monk.sh
• sh -n plugins/monk/hooks/block-monk.sh
• git diff --check