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-16.md

@@ -0,0 +1,17 @@
+---
+id: GAL-16
+title: Enemy Formations
+status: Done
+parent: null
+labels: [enemy, gameplay]
+---
+
+# GAL-16: Enemy Formations
+
+Enemies fly into target positions and hold formation.
+
+## Sub-issues
+
+- [x] [GAL-17](GAL-17.md) — Define target positions for formations (`FormationLayout`).
+- [x] [GAL-18](GAL-18.md) — Enemies have an `Entering` state to fly to their position.
+- [x] [GAL-19](GAL-19.md) — Enemies have an `InFormation` state.