wip: prepare cross-platform previews and workspace improvements

Capture cross-platform startup, SSH credentials, preview packaging gates, and current SSH, answer-reader, math, and icon work. Remove unrelated product comparisons from public source comments.

Preview helper and shell tests pass locally. Native macOS/Linux build, test, and package validation remain pending in CI. Local Windows-only hooks cannot execute under WSL; content and workflow checks were run separately.

packaging/linux/io.github.kuddev.nebula.preview.desktop

@@ -0,0 +1,12 @@
+[Desktop Entry]
+Type=Application
+Name=Nebula Terminal Preview
+GenericName=Terminal Emulator
+Comment=GPU-accelerated terminal for local and remote workflows
+Exec=nebula
+Icon=io.github.kuddev.nebula.preview
+Terminal=false
+Categories=System;TerminalEmulator;
+Keywords=terminal;shell;prompt;ssh;
+StartupNotify=true
+StartupWMClass=Nebula

packaging/linux/io.github.kuddev.nebula.preview.metainfo.xml

@@ -0,0 +1,28 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop-application">
+  <id>io.github.kuddev.nebula.preview</id>
+  <name>Nebula Terminal Preview</name>
+  <summary>GPU-accelerated terminal for local and remote workflows</summary>
+  <metadata_license>CC0-1.0</metadata_license>
+  <project_license>GPL-3.0-or-later</project_license>
+  <developer_name>Kuddev</developer_name>
+  <description>
+    <p>
+      Nebula Terminal combines GPU-accelerated terminal rendering, local and
+      SSH sessions, file workflows, and command-line agent integrations.
+      Preview packages are intended for cross-platform testing before a stable
+      release.
+    </p>
+  </description>
+  <launchable type="desktop-id">io.github.kuddev.nebula.preview.desktop</launchable>
+  <provides>
+    <binary>nebula</binary>
+  </provides>
+  <categories>
+    <category>System</category>
+    <category>TerminalEmulator</category>
+  </categories>
+  <url type="homepage">https://github.com/Kuddev/nebula</url>
+  <url type="bugtracker">https://github.com/Kuddev/nebula/issues</url>
+  <content_rating type="oars-1.1" />
+</component>