Add engine reference docs for SFML3 and Raylib #78

Godot, Unity, and Unreal have full engine reference sets in docs/engine-reference/ (VERSION.md, breaking-changes.md, deprecated-apis.md, current-best-practices.md, modules/). SFML3 and Raylib — despite having engine modules and specialist agents — have zero engine reference docs.

This creates an uneven experience: SFML3/Raylib users get specialist agents but no version-pinned API reference to validate decisions against.

Tasks

• Create docs/engine-reference/sfml3/ with VERSION.md, breaking-changes.md, deprecated-apis.md, current-best-practices.md
• Create docs/engine-reference/raylib/ with same structure
• Pin SFML3 and Raylib versions in AGENTS.md engine configuration
• Update /setup-engine skill to handle SFML3/Raylib reference loading

Affected Files

• docs/engine-reference/sfml3/ (new directory + 4+ files)
• docs/engine-reference/raylib/ (new directory + 4+ files)
• AGENTS.md
• .agents/skills/setup-engine/SKILL.md

Identified during comprehensive framework review on 2026-07-21