feat: emit init-time notice for git extension default change (#2165) (#2432)

Add a non-blocking Panel notice during `specify init` when the git
extension auto-enables, informing users that starting in v0.10.0 this
will require explicit opt-in via `specify extension add git`.

- src/specify_cli/__init__.py: track successful git extension install
  and display yellow "Notice: Git Default Changing" panel
- tests/integrations/test_cli.py: integration test validating notice
  content (v0.10.0 timeline, opt-in messaging, migration command)
- docs/reference/core.md: user-facing NOTE about the upcoming change

Closes #2165