Initial commit: NexusOS project + desktop theme baseline

Captures the known-good state after theme consolidation and
consistency reconciliation:
- NexusOS GTK/xfwm4/icon theme assets (assets/themes, management/Mint-Y-Nexus)
- Tightened right-click menus, visible separators, no menu icons
- assets/themes/install-theme.sh: idempotent restore of all wiring
  (symlinks, xfconf xsettings+xfwm4, GTK 3/4 settings.ini)
- .gitignore excludes venv/ollama/models/runtime/db

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

assets/themes/gtk3-user-overrides.css

@@ -0,0 +1,6 @@
+/* NexusOS — user-level GTK3 overrides.
+   Symlinked to ~/.config/gtk-3.0/gtk.css so it lives with the rest of
+   the NexusOS theme in nexus-core. The theme proper is in NexusOS/. */
+* {
+    caret-color: #8cc63f;
+}