- Add the 0.11→0.12 upgrade path (mix ex_athena.upgrade): a notice-only
  upgrader explaining the TUI/web front-ends are now optional deps (fixes the
  previously-undefined upgrade_0_11_to_0_12/2 reference that broke compilation
  for consumers with igniter).
- Installer (mix ex_athena.install) updates for v0.12.
- Docs: add guides/gemini.md to the docs extras, refresh getting_started, and
  document the installer/upgrader in the CHANGELOG.
- Cover the upgrade + install tasks with tests.