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-38.md

@@ -0,0 +1,17 @@
+---
+id: GAL-38
+title: Challenging Stages
+status: In Progress
+parent: null
+labels: [gameplay, advanced-mechanics]
+---
+
+# GAL-38: Challenging Stages
+
+Special stages every few levels with intricate non-shooting flight patterns and bonus rewards.
+
+## Sub-issues
+
+- [x] [GAL-39](GAL-39.md) — Implement a special stage type (e.g., every 3-4 levels).
+- [ ] [GAL-40](GAL-40.md) — Design and implement intricate flight patterns for enemies that do not shoot.
+- [ ] [GAL-41](GAL-41.md) — Award bonus points for destroying all enemies in the stage.