Add changelog asset to VSIX manifest for VS Code marketplace display

[Copilot]

This PR enables the VS Code marketplace to display the extension's changelog by properly declaring it as an asset in the VSIX manifest.

Problem

The ContextShare extension has a comprehensive CHANGELOG.md file that follows the Keep a Changelog format, but it wasn't being displayed on the VS Code marketplace. The changelog was referenced in documentation but not exposed to the marketplace's asset discovery system.

Solution

Added the changelog asset declaration to vsix/extension.vsixmanifest:

<Asset Type="Microsoft.VisualStudio.Services.Content.Changelog" Path="extension/CHANGELOG.md" />

Verification

• ✅ Built and tested the VSIX package to confirm CHANGELOG.md is included
• ✅ Verified the manifest contains the proper changelog asset declaration
• ✅ Confirmed the existing build process already copies the changelog file to the correct location
• ✅ Validated the changelog follows the Keep a Changelog format expected by VS Code marketplace

The marketplace will now automatically display the extension's version history and release notes, improving user experience and transparency about what changes are included in each release.

Fixes #14.

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