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-9.md

@@ -0,0 +1,16 @@
+---
+id: GAL-9
+title: Scoring
+status: Done
+parent: null
+labels: [gameplay, core-loop]
+---
+
+# GAL-9: Scoring
+
+Add a score resource and increment when an enemy is destroyed.
+
+## Sub-issues
+
+- [x] [GAL-10](GAL-10.md) — Add a `Score` resource.
+- [x] [GAL-11](GAL-11.md) — Increment score when an enemy is hit.