Nixorium v2.0.0-beta.3

Added

• A Raw GitHub install.sh entrypoint that selects a tagged release, prepares
  the private deployment Git repository, and installs the controller from it.

Changed

• Rewrote the deployment upgrade guide with an explicit release example,
  current Nix Flake commands, complete validation, and the Git merge workflow.
• Completed the project-wide Nixorium rebrand across repositories, Flake
  inputs, installer identifiers, command names, desktop identifiers, and the
  Agent Skill.
• Adopted nixorium.org as the canonical website and installer entrypoint,
  with Raw GitHub documented as the bootstrap fallback.

Breaking

• Private deployments must rename their upstream input to nixorium and use
  github:giovantenne/nixorium/<release> when moving to this release.
• Installer environment variables now use the NIXORIUM_ prefix.