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-25.md

@@ -0,0 +1,16 @@
+---
+id: GAL-25
+title: Enemy Variety
+status: In Progress
+parent: null
+labels: [enemy, gameplay]
+---
+
+# GAL-25: Enemy Variety
+
+Different enemy types with distinct visuals, behaviors, and point values.
+
+## Sub-issues
+
+- [x] [GAL-26](GAL-26.md) — `EnemyType` enum (`Grunt`, `Boss`).
+- [x] [GAL-27](GAL-27.md) — Different behaviors, points, and colors based on type.