docs: update CONTRIBUTING, remove wails3 CLI dependency

- Makefile desktop target uses go build directly (no wails3 required)
- CI workflow simplified: go build instead of wails3 build
- CONTRIBUTING: add make install, desktop auth docs, auto-update docs
- CONTRIBUTING: add file listing for icon.png, icon.icns, Taskfile, plist

Makefile

@@ -56,7 +56,7 @@ desktop: ui   ## Build the Wails desktop app for the current platform
 	rm -rf cmd/desktop/frontend/dist
 	mkdir -p cmd/desktop/frontend/dist
 	cp -r web/dist/* cmd/desktop/frontend/dist/
-	cd cmd/desktop && wails3 build
+	cd cmd/desktop && CGO_ENABLED=1 go build -o build/bin/oikos-desktop .
 
 desktop-package: desktop   ## Build + package the desktop app (zip on macOS, tar.gz on Linux)
 	@case $$(uname -s) in \