Add per-version changelog/patch notes to extension push

Problem

When publishing a new extension version with swamp extension push, the only way to communicate what changed is to modify the extension's description field in extension.yaml. This conflates the purpose of the description (a stable summary of what the extension does) with release notes (what changed in this specific version).

For example, I just published a bug fix for an OAuth token cache issue in my Tailscale extension. To note the fix, I had to append it to the description field, which makes the description grow with every release and mixes evergreen information with version-specific notes.

Proposed Solution

Add a changelog or releaseNotes field (either in extension.yaml or as a CLI flag) that attaches version-specific patch notes to a published release without modifying the main description.

Options could include:

• A releaseNotes field in extension.yaml that gets reset each version
• A --release-notes "Fixed OAuth token cache leak" flag on swamp extension push
• A separate CHANGELOG.md file that gets bundled and parsed per-version

The registry could then display these notes per-version, similar to how npm or VS Code extensions show changelogs.

Alternatives Considered

• Appending to description: Works but pollutes the extension summary over time
• Relying on git commit history: Not visible to extension consumers who install from the registry