fix: let users uninstall the Codex plugin from the catalog

[camjac251]

Summary

The Codex marketplace entry declares "installation": "INSTALLED_BY_DEFAULT". Codex reads that as an admin-managed deployment, so the plugin catalog labels GoalBuddy "Installed by admin" and renders a disabled row in place of the uninstall action. A user who installed GoalBuddy themselves cannot remove it from the catalog.

This changes the entry to "AVAILABLE", which is also the enum's default.

Evidence

codex-rs/tui/src/chatwidget/plugin_catalog.rs builds the action row:

if plugin.summary.install_policy == PluginInstallPolicy::InstalledByDefault {
    // ... name: "Installed by admin"
    // ... is_disabled: true
} else if let Some(plugin_id) = plugin_uninstall_id(&plugin.summary) {
    // the uninstall action
}

The same policy also drives the "Admin assigned" and "Enabled by Admin" labels elsewhere in the catalog. It does not trigger auto-install anywhere in core, so the current value buys nothing and costs the uninstall action.

MarketplacePluginInstallPolicy in codex-rs/core-plugins/src/marketplace.rs marks Available as #[default].

"authentication": "ON_INSTALL" is left as is: that is also its enum default.

Test plan

• npm run check passes (109 internal tests).
• The existing marketplace-manifest test asserted the old value, so it is updated with a comment recording why AVAILABLE is correct.