[codex] fix hooks config schema

[bjulian5]

Summary

Removes the unsupported top-level description field from plugins/codex/hooks/hooks.json.

Codex's plugin hook config parser expects the top-level object to contain only hooks, so the released plugin currently fails to parse with:

unknown field description, expected hooks

The descriptive metadata already lives in the plugin manifest; the hook config only needs the hook declarations.

Validation

• jq 'keys' plugins/codex/hooks/hooks.json returns only ["hooks"]
• npm test passes (86/86)

[bjulian5]

@dkundel-openai This PR is ready from my side: it removes the unsupported top-level description key that makes the released plugin hook config fail to parse. Local validation: jq shows only the top-level hooks key, and npm test passes (86/86). GitHub reports the PR as mergeable, but I only have read permission on the upstream repo, so I cannot merge or enable auto-merge.