docs: update changelog for v0.26.0

[augmentmoogi]

Update the v0.26.0 release notes to capture the latest user-facing features, improvements, and fixes.

• Add release notes for PDF conversation attachments, expert bundle auto-archive support, and raw codebase retrieval access
• Document operational and UX improvements, including daemon CLI version visibility, clearer entitlement and permission errors, and removal of deprecated Slack claim/release commands
• Record bug fixes for cloud session sync when targeting a specific agent ID and Git PR/branch sync in worktree-based sessions

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[augmentmoogi]

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