Update CHANGELOG guidance in AGENTS.md - #80848

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"all changes" vs "external changes" is something that we've discussed before with the rest of the @WordPress/gutenberg-components crew, let's make sure we're all aligned here

Also cc @ellatrix

I don't really have strong feelings about this and am happy to go either way.

The way I think about each perspective:

• Always including changelogs is simple, mechanical, and exhaustive. We might not always know what's relevant for our consumers. Organizing sections like discussed in Storybook: Upgrade to 10.5 #80692 (comment) could be an alternative to keep things organized in a way that the most relevant content is surfaced first and the consumer can use their own judgment on whether to skip some content of the changelog that they don't care about.
• Applying judgment in changelogs keeps them entirely relevant and concise. In some ways it saves effort (not having to make the change, especially in a roundabout way after failing CI checks) and in others it introduces effort (in applying judgment about whether a note should be included). It also introduces more risk that changelog notes are skipped when they should in-fact be included.

Closing this as I prefer that we do have more changelogs!