feat: add xnew command with comprehensive run.sh automation

📋 Change summary:
* Added slash-commands/experiments/xnew.md (817 insertions)
* Implements Specification-Driven Development (SDD) framework
* Creates unified scripts/run.sh for Python 3.11 virtual env management
* Includes EARS requirements templates and AI interaction logging
* Supports stack-specific automation (Python, Node.js, Go, Java)
* Integrates nektos/act for local GitHub Actions testing
* Provides complete project initialization with quality gates

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: PaulDuvall