docs: add arena-size OpenSpec change

Plan output-aware arena sizing (~80% of screen) with fallback and resize.

openspec/changes/arena-size/tasks.md

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+# Tasks
+
+## 1. Output handling
+
+- [ ] 1.1 Store output mode dimensions in the output listener.
+- [ ] 1.2 Compute default logical window size as 80% of output divided by scale, with fallback.
+
+## 2. Verification
+
+- [ ] 2.1 Build and run, verifying window is ~80% of screen.
+- [ ] 2.2 Resize window and verify grid updates.