Add GitHub Actions workflow to auto-publish extension on release

[Copilot]

Automates publishing of the Azure DevOps extension to Visual Studio Marketplace when GitHub releases are published.

Changes

• .github/workflows/publish-extension.yml: Workflow that triggers on release publication, builds the task, packages with tfx-cli, and publishes to marketplace using MARKETPLACE_PAT secret
• .github/PUBLISHING.md: Setup instructions for creating and configuring the marketplace PAT token, workflow behavior, and manual publishing fallback

Setup Required

Add MARKETPLACE_PAT repository secret with a Visual Studio Marketplace Personal Access Token (scope: Marketplace Publish). Instructions in .github/PUBLISHING.md.

Workflow

on:
  release:
    types: [published]

Builds task → packages extension → publishes to marketplace automatically on each release.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Create a Github action to publish the task automatically after each release</issue_title>
<issue_description>Create a Github action to publish the task automatically after each release</issue_description>

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

• Fixes Create a Github action to publish the task automatically after each release #22

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