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-28.md

@@ -0,0 +1,19 @@
+---
+id: GAL-28
+title: Boss Galaga & Capture Beam
+status: In Progress
+parent: null
+labels: [enemy, advanced-mechanics]
+---
+
+# GAL-28: Boss Galaga & Capture Beam
+
+Boss enemy with tractor beam capture mechanic and return-to-formation behavior.
+
+## Sub-issues
+
+- [x] [GAL-29](GAL-29.md) — Create a "Boss" enemy type.
+- [x] [GAL-30](GAL-30.md) — Implement the tractor beam attack logic (`boss_capture_attack` system).
+- [x] [GAL-31](GAL-31.md) — Player has a `Captured` component when hit by the beam.
+- [x] [GAL-32](GAL-32.md) — Boss has a `ReturningWithCaptive` state to go back to the formation.
+- [x] [GAL-33](GAL-33.md) — Improve the tractor beam visual effect (currently a simple rectangle).