Add AIDE, Extensify, and Presetify to community extensions (#1961)

* Add AIDE, Extensify, and Presetify to community extensions

Add three extensions from the mnriem/spec-kit-extensions repository:

- AI-Driven Engineering (AIDE): structured 7-step workflow for building
  new projects from scratch with AI assistants
- Extensify: create and validate extensions and extension catalogs
- Presetify: create and validate presets and preset catalogs

Updates both the README community extensions table and
catalog.community.json with entries in alphabetical order.

* fix(tests): isolate preset search test from community catalog growth

Mock get_active_catalogs to return only the default catalog entry so
the test uses only its own cached data and won't break as the
community preset catalog grows.