Add spec-driven development guide

[antonflor]

What changed

• adds docs/development/spec-driven-development.md as a practical, tool-neutral SDD reference
• covers definitions, when to use SDD, core artifacts, the workflow, Git integration, copyable templates, acceptance criteria, a worked example, review checks, and common mistakes
• adds a focused Mermaid lifecycle to docs/visual-guides.md
• indexes the guide and visual from the repository README
• references current official GitHub Spec Kit, Kiro Specs, and Git documentation

Why

The repository explains Git mechanics but did not explain the specification-first workflow that defines requirements, plans implementation, decomposes tasks, and validates completed work. This guide provides a reusable process for both human and AI-assisted development without making Git or a particular agent mandatory.

Style and structure

• follows the current shallow docs/<domain>/<topic>.md layout
• uses required Applies to and Last reviewed metadata
• keeps the diagram in the repository-wide visual guide
• uses lowercase kebab-case naming and official references
• keeps the README change limited to discoverability

Validation

• branch is based on the current main and is not behind
• reviewed against STYLE_GUIDE.md and the documentation-quality workflow requirements
• relative links and Mermaid declarations were checked against the current repository paths
• GitHub Actions will run the repository Markdown structure and offline link checks on this pull request