Add specleft start command (#137)

[Dimwiddle]

Description

Add a new top-level specleft start command for discovery-first onboarding. The command runs health checks, project detection, quick discovery, phase-2 grouping, and a side-by-side code-vs-specs comparison. It stays read-only unless --save is passed.

Type of Change

• New feature
• Documentation update
• Tests

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Testing

• make lint-fix
• make lint
• make test
• make pre-commit

Related Issues

Closes #137