Add Bluesky comments plugin integration

[davidmerfield]

https://emilyliu.me/blog/comments

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Summary

• add a Bluesky comments plugin with entry markup, styling, and registration
• extend blog entry handling and tests for the new comments integration
• document how to enable Bluesky comments from the services and tools guides

Testing

• npm test app/blog/tests/pluginHTML.js (fails: missing scripts/tests/test.env)

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Codex Task