docs: add Lua authoring guide and reorganize docs folder

[amirna2]

Summary

Add a comprehensive Lua authoring guide (docs/lua-authoring-guide.md) — the complete reference for game creators. Covers constructors, rules, conditions, effects, templates, dialogue, built-in verbs, common patterns, and validation errors. Also reorganizes docs/ by moving design documents into docs/design/, adds development invariants to CLAUDE.md, and adds ideas tracking.

Purpose

There was no documentation for someone wanting to write a game in QuestCore. Authors would have to reverse-engineer loader/api.go and the Lost Crown source. This guide makes QuestCore usable by anyone beyond the core developers.

Related plan: docs/plans/game-creator-guide.md

Tests Performed

• All Lua function signatures verified against loader/api.go
• All effect types verified against engine/effects/effects.go
• All template variables verified against engine/effects/effects.go interpolation
• All verb aliases verified against engine/parser/parser.go
• All validation errors verified against loader/validate.go
• Quick Start example verified as a valid game structure that would load and run

Tests Not Performed

• No automated tests — this is a documentation-only change
• make ci not run (no code changes)

CI Checks

• make ci passes locally

AI Assisted

• Yes
• No

🤖 Generated with Claude Code