feat: #3616971 Release Varbase AI Figma 1.0.0-beta1

Bring Varbase AI Figma in line with the Varbase suite conventions and set the README release badge ahead of tagging 1.0.0-beta1.

• Add CHANGELOG.md (Keep a Changelog + Semantic Versioning), backfilled from the 1.0.0-alpha1 and 1.0.0-alpha2 history.
• Add AGENTS.md guidance for AI coding agents, matching the Varbase AI Figma Base recipe's file and adapted for a module.
• Add the Varbase pipeline status, release, and Automated Functional Testing badges to the README, next to the existing Tests badge.
• Add drupalcode to .cspell.json so the new AGENTS.md link text passes the cspell job.

Changes since 1.0.0-alpha2:

• task: #3614568 Add the default .gitlab and .github issue and merge request templates
• task: #3615762 Switch the Varbase functional testing suite to Varbase E2E
• task: #3615762 Update @vardot/varbase-e2e to the latest 2.x

Issue: #3616971

AI-Generated: Yes

Checkpoints:

• File an issue
• Addition/Change/Update/Fix
• Testing to ensure no regression
• Automated unit testing coverage
• Automated functional testing coverage
• UX/UI designer responsibilities
• Readability
• Accessibility
• Performance
• Security
• Developer Documentation
• User Guide Documentation
• Reviewed by human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Release notes snippet
• Release