feat: add print completion-script command

[unicoderbot]

Closes #58

This PR was generated by vgv-ai-bot.

[marcossevilla]

Reviewed the new completion-script command. Ran dart analyze, dart format --set-exit-if-changed, and the full test suite against this branch: all clean, 186 tests passing with 100% coverage. Also verified the error-routing behavior explicitly since it's the crux of this feature: printCompletionScript() sends the script itself through completionLogger.info (stdout), while both CompletionInstallationException and generic exceptions go through completionInstallationLogger.warn/.err (stderr via mason_logger) — so a failure won't corrupt a user's ~/.zshrc when piping with >>. No correctness bugs found, just two minor notes below.

[marcossevilla]

@vgvbot /address-review

[unicoderbot]

On it, I'll address the review comments and push the changes.

• Preparing environment
  • Cloning repository
  • Setting up dev environment
• Reviewing feedback
• Working on changes
• Pushing updates

I've pushed changes to address the feedback.

[marcossevilla]

@vgvbot /address-review

[unicoderbot]

On it, I'll address the review comments and push the changes.

• Preparing environment
  • Cloning repository
  • Setting up dev environment
• Reviewing feedback
• Working on changes
• Pushing updates

I've pushed changes to address the feedback.