Add support for opening posts in the default browser

## Summary

Users should be able to open a selected post directly in their default web browser from the CLI instead of manually copying the URL.

## Requirements

- Add an `open` command to launch a selected post in the user's default browser.
- Display an index for each post when listing results so users can easily identify the post to open.

## Acceptance Criteria

- Users can execute the `open` command to open a selected post.

Start at the CLI entry point and trace how posts are listed and selected. Run the existing listing flow to understand its output, then implement the open command and indexed post display using the selected post URL. Done means a user can identify a listed post and open it in the default browser.

Written by the indexing model from the issue text.