feat(architecture): extract code2flow package for Mermaid diagrams and visualizations

[tom-sapletta-com]

Summary

• Extracted Mermaid flow diagrams, call graph visualizations, and syntax sanitizers into standalone package code2flow (v0.1.0) under packages/code2flow.
• code2flow provides:
  • Mermaid exporters: export_compact, export_flow_compact, export_flow_detailed, export_flow_full, export_calls.
  • Mermaid validation and repair: validate_mermaid_file, fix_mermaid_file (supporting both str and Path).
  • Clean dependency on pure topology package code2graph without coupling to AST parsers or file system scanners.
• Added code2flow to pyproject.toml optional dependencies ([project.optional-dependencies] diagrams) and dev dependencies.
• Verified with 362/362 pytest tests passing.