fix(plugins): restore list defaults and remove turtle import

[ruizanthony]

Summary

• Restore plugin list API defaults so calls without an explicit filter return both custom and builtin plugins.
• Remove accidental from turtle import stamp import from the plugin installer helper, which imports Tkinter and breaks headless/Docker environments without _tkinter.

Validation

• python -m py_compile api/plugins_list.py plugins/_plugin_installer/helpers/install.py plugins/_plugin_installer/api/plugin_install.py
• Smoke-tested dynamic loading of plugins/_plugin_installer/api/plugin_install.py.
• Smoke-tested import of api.plugins_list.PluginsList.

Context

On current origin/main, /api/plugins_list defaults both custom and builtin to False, so an empty or missing filter returns an empty plugin list. Also the plugin installer endpoint can fail on import in headless environments because turtle imports tkinter, which requires _tkinter.