chore(todo): split TODO.md into per-issue files under TODO/

Convert the flat TODO.md into a TODO/ folder where each GAL-N issue is
its own Linear-style markdown document with YAML frontmatter (id, title,
status, parent, labels), and add a README index grouped by section.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

TODO/GAL-12.md

@@ -0,0 +1,17 @@
+---
+id: GAL-12
+title: Levels/Stages
+status: Done
+parent: null
+labels: [gameplay, core-loop]
+---
+
+# GAL-12: Levels/Stages
+
+Stage progression with increasing difficulty driven by configuration resources.
+
+## Sub-issues
+
+- [x] [GAL-13](GAL-13.md) — Add `CurrentStage` and `StageConfigurations` resources.
+- [x] [GAL-14](GAL-14.md) — Define criteria for clearing a stage.
+- [x] [GAL-15](GAL-15.md) — Advance to the next stage with increasing difficulty.