feat: allow CLI users to disable completion auto-install

[unicoderbot]

Warning

The coding agent reported this implementation is incomplete.
Code and tests are fully implemented, but analyze/tests could not be run because the provided Flutter SDK (Dart 3.12.2) is older than the repo's required ^3.13.0.

This PR has been opened as a draft. Review the diff against the original issue before marking it ready for review.

Summary

Adds the ability for CLI users (not just CLI maintainers) to opt out of the automatic installation of completion files.

What changed

• New hidden command disable-completion-auto-install (DisableCompletionInstallationCommand), added to CompletionCommandRunner only when enableAutoInstall is enabled. It is also registered as a reserved command so running it does not itself trigger an auto install.
• Persisted preference: CompletionConfiguration gains an enabled flag that is serialized to/from config.json (enabled JSON key). It defaults to true when missing or of an invalid type, so existing configurations keep working.
• CompletionInstallation exposes isAutoInstallEnabled and setAutoInstallEnabled({required bool enabled}) to read/write the persisted preference, and _shouldInstall now short-circuits when the user has disabled auto installation.
• Re-enabling: manually running install-completion-files writes enabled: true again, since installing implies the user wants completion.
• Docs: README documents the new opt-out command and how to re-enable.
• Tests: coverage for the new command registration/behavior, configuration serialization/copyWith, and the installation enable/disable logic (plus some formatting cleanups in existing tests).

Closes #55