Label plugins as plugins, and hide skills/apps for given plugin

[canvrno-oai]

• Updates $-mention behavior for plugins: plugin-installed skills and plugin-backed apps are hidden from dollar-sign autocomplete, so the user-facing target is the plugin rather than its skills or apps.
• Adds a test for the plugin-owned collision case where a plugin, one of its skills, and one of its apps share the same visible name. The test verifies that the mention menu collapses to a single [Plugin] entry rather than separate skill or app entries.
• Updates both tui and tui_app_server with the same changes