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/learning_linux_test.go

@@ -127,7 +127,7 @@ func TestParseStraceLog(t *testing.T) {
 	}, "\n")
 
 	logFile := filepath.Join(t.TempDir(), "strace.log")
-	if err := os.WriteFile(logFile, []byte(logContent), 0o644); err != nil {
+	if err := os.WriteFile(logFile, []byte(logContent), 0o600); err != nil {
 		t.Fatal(err)
 	}