Show a concise “What’s New” summary after copilot-cli updates

[myartsev]

Describe the feature or problem you'd like to solve

Today, after updating Copilot CLI, I have to manually go read GitHub release notes to understand what changed. This interrupts my workflow and makes it easy to miss new commands, flags, or behavior changes that could immediately affect how I use the CLI.

Proposed solution

After upgrading Copilot CLI, the CLI could optionally display a short summary of what changed since the previous installed version.

For example:

• New commands or subcommands
• Behavior changes or breaking changes
• Notable improvements or fixes

This could be:

• Automatically shown once after an update
• Or available via a command like /whats-new or /changelog

This would let users immediately adjust their workflow without leaving the terminal.

For example:

✔ Copilot CLI updated (v1.12 → v1.13)

What’s new:
- Added `copilot explain --diff`
- Improved shell history context handling
- Fixed auth refresh issues on macOS

Run `copilot whats-new` anytime to see this again.

Example prompts or workflows

No response

Additional context

No response