[CI] Add Meshery UI integration testing for pull requests

[Copilot]

Adds automated integration testing that validates Sistent changes against Meshery UI on every PR, catching breaking changes before merge.

Changes

New workflow: .github/workflows/test-meshery-integration.yml

• Builds Sistent from PR, packages as tarball via npm pack
• Installs into Meshery UI (meshery/meshery repo)
• Runs Meshery UI build and tests to verify compatibility
• Reports results in PR summary with conditional success/failure messaging

Documentation: README.md

• Added "Continuous Integration" section explaining automated integration testing

Technical Details

• Uses Node.js 20 with npm caching for performance
• Explicit permissions: contents: read for security
• Error handling for tarball detection and test script presence
• Precise regex (grep -E '"test"\s*:') to detect test scripts
• Excludes only system/** and *.md from triggering to allow workflow changes to be tested

This mirrors the documented local testing approach (npm pack → install → build) ensuring CI/local consistency.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[CI] Use Meshery UI as a sample application in which to test changes in pull requests</issue_title>
<issue_description>#### Current Behavior

Desired Behavior

Implementation

Acceptance Tests

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Contributor Guides and Resources

• 🛠 Meshery Build & Release Strategy
• 🎨 Wireframes and designs for Sistent site in Figma (open invite)
• 🙋🏾🙋🏼 Questions: Layer5 Discussion Forum and Layer5 Community Slack
  </issue_description>

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

• Fixes [CI] Use Meshery UI as a sample application in which to test changes in pull requests #1221

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