[FEATURE] Add /release-notes built-in command for viewing Codex CLI changelog

What feature would you like to see?

A /release-notes slash command that displays the latest updates and changes in Codex CLI, similar to how Claude Code implements this feature.

Problem it solves:

Currently, users have no built-in way to quickly view what's new in Codex CLI without leaving the terminal to check the GitHub releases page or changelog. This creates friction when:

• Trying to understand what new features are available after an update
• Checking if a specific bug was fixed in the current version
• Staying informed about breaking changes or deprecations

Proposed solution:

Implement a /release-notes command that:

1. Fetches and displays recent release notes (last 3-5 versions by default)
2. Shows the current installed version prominently
3. Highlights new features, bug fixes, and breaking changes
4. Optionally filters to show only notes since the last version the user was on

Reference implementation:

Claude Code has a similar feature that serves as a good model for UX. Running /release-notes in Claude Code shows a well-formatted summary of recent changes, making it easy to discover new capabilities.

<img width="411" height="91" alt="Image" src="https://github.com/user-attachments/assets/75663811-1da2-4b25-8c97-ff20f6aa7550" />

!Image

Additional information

• Current Codex CLI version: codex-cli 0.84.0
• Platform: Darwin 25.2.0 arm64 arm

This would improve discoverability of new features and help users stay up to date with the latest improvements.