fix: resolve all golangci-lint v2 warnings (29 issues)

Migrate to golangci-lint v2 config format and fix all lint issues:
- errcheck: add explicit error handling for Close/Remove calls
- gocritic: convert if-else chains to switch statements
- gosec: tighten file permissions, add nolint for intentional cases
- staticcheck: lowercase error strings, simplify boolean returns

Also update Makefile to install golangci-lint v2 and update CLAUDE.md.

internal/sandbox/linux_features.go

@@ -370,7 +370,7 @@ func suggestInstallCmd(features *LinuxFeatures) string {
 }
 
 func readSysctl(name string) string {
-	data, err := os.ReadFile("/proc/sys/" + name)
+	data, err := os.ReadFile("/proc/sys/" + name) //nolint:gosec // reading sysctl values - trusted kernel path
 	if err != nil {
 		return ""
 	}