Add /changelog slash command

[SteveXCIV]

Describe the feature or problem you'd like to solve

Add a new slash command to display the changelog/release notes in a CLI session.

Proposed solution

Since Copilot CLI now downloads new versions automatically, it would be convenient to be able to run a slash command to view the release notes right in a session.

Example prompts or workflows

No response

Additional context

Mockup:

(assume v0.0.399 is running)

/changelog

## 0.0.399 - 2026-01-29

- Compaction messages show clearer command hints to view checkpoint summaries
- Press Ctrl+X then / to run slash commands without losing your input
... (truncated for brevity) ...