refactor: implement component-based architecture for enhanced maintainability

- Reorganize directory structure into core/, features/, and themes/
- Colocate application Nix logic, configs, scripts, and theme overrides
- Implement 'Inversion of Control' for theming: apps now pull theme-specific layouts
- Update flake.nix and shared library paths to match the new structure
- Document the new Feature-Centric architecture in README.md

features/apps/opencode/config/opencode.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "https://opencode.ai/config.json",
+  "theme": "system",
+  "autoupdate": false
+}