docs: add arena-size OpenSpec change

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

openspec/changes/arena-size/specs/arena-sizing/spec.md

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+# Spec Delta
+
+## Purpose
+
+Defines how the arena default size is derived from the output and how resizing affects the grid.
+
+## ADDED Requirements
+
+### Requirement: Default arena is about 80% of output
+The system SHALL compute the default logical window size as 80% of the output mode dimensions divided by the active scale.
+
+#### Scenario: Output available
+- **WHEN** output mode and scale are known at startup
+- **THEN** the default logical window size is approximately 80% of the output
+
+### Requirement: Fallback when output unknown
+The system SHALL use a fallback default size when output info is unavailable.
+
+#### Scenario: No output info
+- **WHEN** output mode is not reported
+- **THEN** a fallback logical size is used
+
+### Requirement: Cell size remains 8 logical pixels
+The system SHALL keep an 8 logical pixel cell size.
+
+#### Scenario: Grid dimensions
+- **WHEN** the logical window size is known
+- **THEN** grid columns and rows are logical size divided by 8
+
+### Requirement: Resizable window
+The system SHALL allow the window to be resized by the user.
+
+#### Scenario: Resize
+- **WHEN** the user resizes the window
+- **THEN** the grid dimensions update accordingly
+
+### Requirement: Grid preserved on resize
+The system SHALL preserve overlapping cells when the grid dimensions change.
+
+#### Scenario: Enlarge
+- **WHEN** the window grows
+- **THEN** existing cells within the old bounds remain