Add per-workspace layout configuration with smart auto-balancing:
**Core Changes:**
- Add LayoutConfig interface to Workspace type with insertionMode, splitPercentage, insertionPosition
- Create layout-utils.ts with smart direction algorithm that auto-balances horizontal/vertical splits
- Update addWindow() to use workspace layoutConfig instead of hardcoded values
- Migrate state from v7→v8, adding layoutConfig to all workspaces with smart defaults
**Smart Direction Algorithm:**
- Analyzes layout tree to count horizontal vs vertical splits
- Automatically balances by favoring the less-common direction
- Defaults to horizontal (row) for first split
- Provides foundation for "Balanced (auto)" insertion mode
**Testing:**
- Add 30 comprehensive tests for layout-utils.ts (tree analysis, smart direction, window insertion)
- Add 30 tests for logic.ts addWindow() with different layout configs (row/column/smart modes)
- Update migration tests to verify v6→v7→v8 and v7→v8 paths
**Migration:**
- v7→v8 adds layoutConfig with defaults: smart mode, 50% split, second position
- All existing workspaces automatically get smart auto-balancing behavior
Implements orthogonal design: layout behavior = workspace settings (not command flags)
