ci: add focused Claude workflows with context-aware prompts

Add claude-mention.yml for @claude mentions in issues/PRs and
claude-pr-review.yml for automatic PR reviews.

Key improvements:
- claude-mention: Triggers on @claude with project-specific prompt
- claude-pr-review: Auto-reviews PRs from new contributors with CONTRIBUTING.md criteria
- Both workflows: Restricted permissions (read-only contents, no merge capability)

🤖 Generated with Claude Code (https://claude.ai/code)
Co-Authored-By: Claude <[email protected]>