Add autocompletion for flags

[draftcode]

https://github.com/spf13/cobra/blob/main/site/content/completions/_index.md#specify-dynamic-flag-completion

Cobra supports autocompletion for args and flags. We can provide better completion candidates for these two:

• av stack branch: --parent should have branches as candidates
• av stack sync: --parent should have branches as candidates

(The autocompletion can be enabled with https://docs.aviator.co/aviator-cli/how-to-guides/setup-auto-completion)