fix(dx): use coreerr.E() and go-io, update CLAUDE.md, add tests
- Replace 90+ fmt.Errorf calls with coreerr.E() from go-log across
display, window, systray, keybinding, contextmenu, and mcp packages
- Replace os.ReadFile/WriteFile/MkdirAll with coreio.Local in
window/layout.go and window/state.go
- Update CLAUDE.md: fix key files table for new package structure,
document error handling and file I/O conventions, add missing deps
- Add 37 tests for window package (task handlers, persistence,
tiling modes, snap positions, workflow layouts)
- Window coverage: 47.1% → 69.8%
Co-Authored-By: Virgil <virgil@lethean.io>
b559562dd9