Bootstrap NebulaOS repo and desktop shell UI

Initialize the NebulaOS monorepo layout with top-level documentation, ignore rules, placeholder service/package directories, compositor docs plus a baseline Sway config, and session startup/desktop entry files. Add the first desktop shell prototype under `shells/desktop/ui` using React + Vite, including themed surface components (desktop, top bar, launcher, preview), mock Nebula APIs and shell bridge state wiring, reusable UI controls/icons, and supporting styles/config needed for local development.

session/nebula-session

@@ -0,0 +1,10 @@
+#!/bin/sh
+# NebulaOS session startup
+
+set -e
+
+export XDG_CURRENT_DESKTOP=NebulaOS
+export XDG_SESSION_DESKTOP=nebula
+
+# Shell and services are wired here as they land
+# exec nebula-shell-desktop

session/nebula.desktop

@@ -0,0 +1,6 @@
+[Desktop Entry]
+Name=NebulaOS
+Comment=NebulaOS desktop session
+Exec=nebula-session
+Type=Application
+DesktopNames=NebulaOS