docs: Document Articles IV, V, VI as project-customizable in spec-driven.md

### Problem

`spec-driven.md` states "The constitution defines nine articles" but only documents six (I, II, III, VII & VIII combined, IX). Articles IV, V, and VI are entirely missing, creating a confusing gap for users and agents trying to understand or verify constitutional compliance.

The constitution template (`templates/constitution-template.md`) has placeholder slots for these with example hints (`Integration Testing`, `Observability`, `Versioning & Breaking Changes`), and the `/speckit.analyze` command dynamically validates against whatever a project defines — so the system already treats IV–VI as project-customizable. But `spec-driven.md` doesn't explain this.

Related: #1388, #310

### Proposed fix

Add general descriptions for Articles IV, V, and VI to the "Nine Articles of Development" section in `spec-driven.md` that:

1. **Describe the concern** each article addresses at a general level (e.g., "Article IV addresses security and access boundaries")
2. **Explicitly state** that the concrete rules are project-defined via the constitution template
3. **Reference the template placeholders** so users know where to customize

This preserves the intentional flexibility (proven by the Salesforce extension defining its own IV–VI) while closing the documentation gap. The "nine articles" framing stays accurate and the analyze command's dynamic validation is unaffected.