docs: update changelog for v0.27.0

[augmentmoogi]

Update the changelog with the v0.27.0 release notes to document the latest product capabilities, usability improvements, and bug fixes.

• Add release notes for new features including Cosmos handoff, Warp integration, expanded webhook support, plugin marketplace branch/tag selection, hook enhancements, subagent completion reporting, daemon pool status commands, and CLI session history access for ACP integrations.
• Summarize usability and reliability improvements across session browsing, pasted text handling, cloud agent resource loading, connection stability, shared session access control alignment, environment management, expert configuration, workspace flag shortcuts, and Cosmos terminology.
• Capture bug fixes for webhook routing and subscriptions, header-auth MCP secrets in daemon mode, unnecessary empty session creation after connection failures, stream anchor rendering, and expert include fallback behavior.

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[augmentmoogi]

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