docs: #3633859 Add a Claude Code agent and skill for applying and verifying Varbase AI Default

Adds a Claude Code agent and skill to the recipe, so a coding agent can apply, repair and verify Varbase AI Default on a Varbase 11 site.

Issue: https://www.drupal.org/i/3633859

• .claude/agents/varbase-ai-base.md: the agent, its hard rules (DDEV only, snapshot first, no secrets) and its apply, repair and verify modes.
• .claude/skills/varbase-ai-base/SKILL.md: the commands, post-apply steps, checks and traps, read from the recipe code.

Documentation only: no runtime code or config change.

AI-Generated: Yes. Drafted with Claude Code; not yet reviewed by a human.

Checkpoints

• Testing to ensure no regression
• Automated unit testing coverage
• Automated functional testing coverage
• UX/UI designer responsibilities
• Readability
• Accessibility
• Performance
• Security
• Documentation
• Reviewed by a human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Release