Assert hook-free Codex manifests in pack-install smoke test

[chenliuyun]

Context

Follow-up from #73. The PR correctly removes the unsupported Codex onInstall hook configuration and is safe to merge, but the pack-install smoke test only stops requiring .codex-plugin/hooks.json; it does not assert that the published tarball is hook-free.

This is a non-blocking regression-test improvement.

Problem

scripts/smoke-codex-pack-install.mjs can still pass if a future packaged artifact reintroduces either:

• a hooks field in a Codex plugin manifest, or
• .codex-plugin/hooks.json

The smoke test should verify both supported installation layouts after packing and installing the tarball.

Acceptance criteria

• Assert the installed package-root .codex-plugin/plugin.json does not contain hooks.
• Assert the installed package-root .codex-plugin/hooks.json does not exist.
• Assert the nested plugins/switchbot/.codex-plugin/plugin.json does not contain hooks.
• Assert the nested plugins/switchbot/.codex-plugin/hooks.json does not exist.
• Keep the existing source-level regression tests and pack-install smoke test passing.

Related: #73