[codex] Add Android emulator QA plugin

[vb-openai]

Summary

• add a new android plugin that vendors the android-emulator-qa skill from openai/skills-internal
• register the plugin in .agents/plugins/marketplace.json with the Coding category
• fix plugin-creator so its default marketplace path points at .agents/plugins/marketplace.json and its scaffolded defaultPrompt matches the current loader contract
• update the plugin-creator reference spec to match the fixed scaffold shape

Why

The goal of this change is to convert the existing Android emulator QA skill into a first-class plugin in this repo and verify that the plugin loads the way Codex actually consumes plugins today.

While testing the conversion, two scaffold issues surfaced:

• plugin-creator was writing marketplace output to .agents/skills/plugins/marketplace.json instead of the repo marketplace file
• the scaffold emitted interface.defaultPrompt in a shape that the installed Codex loader rejected for this plugin path

This PR fixes those issues alongside the new plugin so the conversion path is reproducible.

Validation

• python3 -m py_compile .agents/skills/plugin-creator/scripts/create_basic_plugin.py plugins/android/skills/android-emulator-qa/scripts/ui_pick.py plugins/android/skills/android-emulator-qa/scripts/ui_tree_summarize.py
• JSON parse check for plugins/android/.codex-plugin/plugin.json, plugins/android/plugin.lock.json, and .agents/plugins/marketplace.json
• git diff --check
• byte-for-byte parity check between plugins/android/skills/android-emulator-qa/* and /Users/vb/Documents/skills-internal/.codex/skills/android-emulator-qa/*
• isolated CODEX_HOME smoke test showing Codex app-server skills/list discovers android:android-emulator-qa from the installed plugin
• plugin-creator smoke run in a temp directory verifying the corrected marketplace path and string defaultPrompt scaffold output

[gverma-openai]

This is incorrect, defaultPrompt is an array of strings.