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.go

@@ -41,7 +41,7 @@ func ParseStraceLog(logPath string, debug bool) (*StraceResult, error) {
 	if err != nil {
 		return nil, fmt.Errorf("failed to open strace log: %w", err)
 	}
-	defer f.Close()
+	defer func() { _ = f.Close() }()
 
 	home, _ := os.UserHomeDir()
 	seenWrite := make(map[string]bool)