docs: update changelog for v0.28.0

[augmentmoogi]

Update the release notes for v0.28.0.

• Add the new general-purpose subagent capability for broader multi-step research and implementation workflows.
• Document key usability and reliability improvements, including configurable default model behavior, visible markdown link destinations, daemon update warnings, clearer cloud startup errors, improved VFS diagnostics, and nested lazy skill includes.
• Note the cloud-agent fix for retrying transient knowledgebase sync failures and shutting down cleanly on unrecoverable errors.

---

🤖 This description was generated automatically. Please react with 👍 if it's helpful or 👎 if it needs improvement.

[github-actions]

Review completed. No suggestions at this time.

[augmentmoogi]

👋 This PR is being monitored by an automated agent. Here's what I can do:

• Answer questions about the changelog changes
• Implement suggestions if you request formatting or content changes
• Fix merge conflicts if the PR falls behind main
• Merge the PR once a human has approved it

I check in every 5 minutes. Any new comment will wake me up.