docs: update changelog for v0.27.1

[augmentmoogi]

Update v0.27.1 release notes to document the new built-in general-purpose sub-agent.

• Describe support for a built-in general-purpose sub-agent designed for deep research, code search, and multi-step workflows.
• Capture the release impact so reviewers and users can quickly understand what was introduced in v0.27.1.

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[augmentmoogi]

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[github-actions]

Thanks for updating the release notes. I left one documentation consistency comment to help keep the changelog aligned with published stable versions.

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[github-actions]

Because this new top entry now jumps straight from 0.27.1 to 0.25.1, the changelog reads as if stable releases 0.26.0 and 0.27.0 never happened, but both tags already exist in the repo. Could we add those missing sections (or otherwise clarify why they’re intentionally omitted) so users aren’t missing two stable releases worth of notes?