[Bug] Desktop app: after adding a GitHub plugin marketplace, it can't be found or installed

Adding a plugin marketplace from GitHub via the **Desktop app** appears to be broken. Reproduced with:

`https://github.com/makenotion/claude-code-notion-plugin.git`

After adding this marketplace through the Desktop app:
- `~/.claude/plugins/known_marketplaces.json` and `extraKnownMarketplaces` in `settings.json` correctly record the marketplace.
- The marketplace source is checked out locally under `~/.claude/plugins/marketplaces/notion-plugin-marketplace/`, and the plugin files are already present in `~/.claude/plugins/cache/notion-plugin-marketplace/notion-workspace-plugin/0.1.0/`.
- But in the Desktop app UI itself, I cannot find the marketplace anywhere, searching for it or its plugin by name returns nothing, and there is no UI to install/enable a plugin from it.
- `enabledPlugins` in `settings.json` never gets an entry for the plugin, even though the plugin files are already cached locally.

The terminal's `/plugin` menu at least lists this marketplace (it reads `known_marketplaces.json` directly from disk), but doesn't complete installation/enabling either. So the practical effect is: adding a marketplace via the Desktop app never results in a usable, enabled plugin, in either the Desktop app or the terminal.

**Environment**: macOS 15.7.3 (24G419), Claude Code version 2.1.263 (per `~/.claude.json` record, may not be exact).

**Expected behavior**: After adding a marketplace via the Desktop app, it should be visible/searchable in the app, with a working UI to install and enable its plugins.

Reproduce on macOS by adding the GitHub marketplace URL in the Desktop app, then compare what appears in ~/.claude/plugins/known_marketplaces.json, settings.json, the marketplaces checkout, and the plugin cache. Also check the terminal /plugin menu behavior. Done means the marketplace is visible/searchable in the app and its plugin can be installed and enabled, with enabledPlugins updated.

Written by the indexing model from the issue text.