Zombie builtin themes

This came up in the course of preparing workshop materials.

Many of the themes seen in the Themes section of the Extensions View when you filter for @builtin themes aren't actually available if you do Preferences: Color Theme from the command palette. Examples include Abyss or Monokai Dimmed. This is kind of confusing, if you're looking over the builtin extensions.

Apparently we intentionally exclude these themes:

https://github.com/posit-dev/positron/blob/e8471b543a76af57f04f328e5f1f0813d378f5dc/src/vs/workbench/services/themes/browser/workbenchThemeService.ts#L384-L400

Should we exclude them more systematically? E.g. literally remove them or maybe disable them in some more profound way that prevents them from appearing in our @builtin extensions.

Screenshots made with:
Positron Version: 2025.07.0 (Universal) build 88
Code - OSS Version: 1.100.0
Commit: 27dd8feb5758b5ccecb38b33cbdb0c034c67d786
Date: 2025-06-12T03:34:40.909Z
Electron: 34.5.1
Chromium: 132.0.6834.210
Node.js: 20.19.0
V8: 13.2.152.41-electron.0
OS: Darwin arm64 24.5.0

Start with src/vs/workbench/services/themes/browser/workbenchThemeService.ts around the linked exclusion logic. Reproduce the mismatch by comparing the @builtin Themes results with Preferences: Color Theme, then trace how each list is populated. Done means themes unavailable from the command palette no longer appear as builtin extensions, with the behavior verified in both views.