Fix invalid Codex hooks configuration

[shuto-S]

Summary

• remove the unsupported onInstall hook configuration from both Codex plugin layouts
• keep CLI and credential setup explicit through switchbot codex setup
• update regression tests and the pack-install smoke test for the hook-free manifests
• document the distinct Claude Code and Codex setup strategies in CONTRIBUTING.md

Root cause

Codex hook config accepts description and hooks at the top level. The plugin shipped an onInstall top-level field, so Codex rejected the file with:

unknown field `onInstall`, expected `description` or `hooks`

Codex does not provide an install-time lifecycle event that can safely replace this. Running the setup helper from SessionStart would repeat plugin registration on every session, so this change removes the invalid hook and keeps the existing explicit setup flow.

Validation

• npm run verify:pre-commit
• npm run verify:release-gate with isolated CODEX_HOME and npm cache
• npx markdownlint CONTRIBUTING.md packages/codex-plugin/README.md
• installed the local marketplace and plugin with Codex CLI 0.146.0 using an isolated CODEX_HOME; switchbot@switchbot loaded without the hook parse warning