fix: correct constitution path references and cleanup orphaned commands

- validate.md: Update 5 references from constitution.md to engineering-principles.md
- heal-workflow.md: Update 4 references from .spec-flow/memory/constitution.md to docs/project/engineering-principles.md
- CLAUDE.md: Add /constitution to quick reference Project Setup section
- Delete orphaned update-project-config.md command
- Move implement-epic-legacy.md to _archived
- Move announce-release.md and studio.md to internal/

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

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: marcusgoll