Move legacy docs to project root docs/legacy

Relocated SYSTEM_REVIEW.md and PROJECT_PLAN_ENERGY_EVOLUTION.md from
backend/docs/legacy to project-level docs/legacy. Added docs/README.md
indexing all documentation including ARCHITECTURE.md and GAME_RULES.md.

backend/docs/README.md

@@ -1,13 +0,0 @@
-# Backend Documentation
-
-This directory contains documentation for the Mantimon TCG backend.
-
-## Directory Structure
-
-### `/legacy/`
-Historical documents from completed development phases. Preserved for reference.
-
-| File | Description |
-|------|-------------|
-| `SYSTEM_REVIEW.md` | Comprehensive code review of the core engine (Jan 2026). All 15 identified issues resolved. 826 tests passing. |
-| `PROJECT_PLAN_ENERGY_EVOLUTION.md` | Implementation plan for energy/tool attachment and evolution stack refactor. Completed Jan 2026. |

docs/README.md

@@ -0,0 +1,17 @@
+# Mantimon TCG Documentation
+
+## Active Documentation
+
+| File | Description |
+|------|-------------|
+| `ARCHITECTURE.md` | Technical architecture overview covering frontend (Vue+Phaser), backend (FastAPI), database schema, real-time communication, game engine design, and offline fork considerations. |
+| `GAME_RULES.md` | Game rules document defining campaign structure, base ruleset, energy system, deck building, win conditions, turn structure, card types, and status conditions. |
+
+## Legacy Documentation
+
+Historical documents from completed development phases, preserved for reference.
+
+| File | Description |
+|------|-------------|
+| `legacy/SYSTEM_REVIEW.md` | Comprehensive code review of the core game engine (Jan 2026). Identified 15 issues across models, effects, and engine - all resolved. 826 tests passing. |
+| `legacy/PROJECT_PLAN_ENERGY_EVOLUTION.md` | Implementation plan for energy/tool attachment refactor and evolution stack system. Changed `attached_energy`/`attached_tools` from `list[str]` to `list[CardInstance]`. Completed Jan 2026. |