chore: add agentic first-time contributor workflow

[Junnygram]

Description

This PR replaces the legacy (or missing) newcomer welcoming logic with a modern, agentic workflow using the gh-aw framework. This is part of the effort to standardize and harden newcomer onboarding across the Meshery ecosystem.

Key Features:

• Personalized Onboarding: The assistant analyzes the PR content and provides tailored resource links based on the specific files touched (e.g., schemas vs. documentation).
• Hardened Bot Filter: Includes a production-grade filter to ignore Dependabot, Copilot, and other automated agents, reducing notification noise.
• DCO Compliance: Automatically checks for commit sign-offs and provides immediate fix instructions if they are missing.
• Security: Hardened with checkout: false to prevent untrusted code execution during the pull_request_target event.

Next Steps

This workflow is being evaluated for its effectiveness in providing high-quality, relevant guidance to first-time contributors.

[github-actions]

Preview deployment for PR #7723 removed.

This PR preview was automatically pruned because we keep only the 6 most recently updated previews on GitHub Pages to stay within deployment size limits.

If needed, push a new commit to this PR to generate a fresh preview.