docs: add arena-size OpenSpec change

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

openspec/changes/arena-size/proposal.md

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+# Proposal
+
+## Why
+
+The current default arena is 400×400 logical pixels (50×50 cells), too small for comfortable exploration. The arena should occupy about 80% of a FullHD screen, adapting to the actual output size and scale.
+
+## What Changes
+
+- Use `wl_output` mode dimensions to compute a default logical window size equal to 80% of the output, divided by the active scale.
+- Keep the 8 px logical cell size; grid dimensions derive from the logical window size.
+- Preserve resizable behavior and grid recalculation on resize.
+- Provide a sensible fallback when output info is unavailable.
+
+## Capabilities
+
+### New Capabilities
+
+- `arena-sizing`: output-aware default arena size (~80% of screen) with fallback and resize handling.
+
+### Modified Capabilities
+
+None.
+
+## Impact
+
+- Affected code: `wlgol.c` (output mode handling, default size calculation, canvas/grid resize).
+- No new dependencies.