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/LICENSE

@@ -0,0 +1,23 @@
+NexusOS theme assets — attribution and license
+================================================
+
+The NexusOS GTK2 and xfwm4 themes are derived from Mint-Y-Dark-Aqua,
+part of the mint-themes package shipped by Linux Mint.
+
+Upstream:
+  https://github.com/linuxmint/mint-themes
+  Copyright (c) 2017-2024 Linux Mint <root@linuxmint.com>
+  Licensed under the GNU General Public License v3.0 or later.
+
+Derivative work in this directory:
+  - NexusOS/gtk-2.0/        (PNGs recolored from Mint-Y-Dark-Aqua/gtk-2.0/)
+  - NexusOS/xfwm4/          (PNGs recolored from Mint-Y-Dark-Aqua/xfwm4/)
+  - NexusOS-gtk2-src/       (build script that performs the recolor)
+  - NexusOS-xfwm4-src/      (build script that performs the recolor)
+
+As a derivative of GPLv3 material, these files are likewise distributed
+under the terms of the GNU General Public License version 3 or later. See
+https://www.gnu.org/licenses/gpl-3.0.html for the full license text.
+
+The NexusOS GTK3 theme (NexusOS/gtk-3.0/) and the NexusOS icon set
+(NexusOS-icons*/) are original works and not derived from Mint-Y.