Add 0.6.1 release notes

[Copilot]

Creates release notes for version 0.6.1 based on the cherry-picked changes from commit 943ba6c.

Changes Documented

• Build & Packaging: Version file support via VERSION.txt with MANIFEST.in inclusion (Getting package version from file when building packages #261, Adding MANIFEST.in file to include package version files in builds #263)
• Bug Fix: OAuth _SignInState serialization refactored to use Pydantic BaseModel (Fixing _sign_in_state serialization/deserialization #264)

File Added

• release-notes-0.6.1.md — Follows existing release notes format with overview, affected libraries, upgrade instructions, and support resources

Original prompt

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This section details on the original issue you should resolve

<issue_title>0.6.1 Release Notes</issue_title>
<issue_description>Based on the release notes found here: https://github.com/microsoft/Agents-for-python/releases

Create release notes for 0.6.1

This should include the cherry-picked changes found in this commit: 943ba6c

This format should be Markdown. Respond to this issue.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes 0.6.1 Release Notes #271

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