[codex] align Codex plugin install config

[tzachbon]

Summary

• Replace old Codex install docs with marketplace sparse install.
• Switch hook docs and manifest to plugin_hooks plus hooks/hooks.json.
• Add marketplace auth policy and sync Ralph versions to 4.10.2.

Checks

• jq touched JSON
• tests/helpers/version-sync.sh
• bats tests/codex-plugin.bats
• git diff --check
• codex features list shows plugin_hooks true
• origin/main is ancestor of branch HEAD

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• Commit unit tests in branch codex-update-2026-05-19

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