test(cmd): add comprehensive test coverage for command package

Add test suites achieving >90% coverage for cmd/ package:
- cmd/fmt_test.go: Format command logic, check mode, exit handling
- cmd/lint_test.go: Lint command with all component types
- cmd/root_test.go: Root command flags and execution
- cmd/subcommands_test.go: All subcommand routing
- cmd/summary_test.go: Summary display formatting

Refactor cmd/fmt.go and cmd/root.go to use exitFunc variable for
testability, allowing tests to override os.Exit behavior.

Author: Test User

cmd/fmt.go

@@ -216,7 +216,7 @@ func runFmt(args []string) error {
 
 	// Check mode: exit 1 if files need formatting
 	if fmtCheck && len(needsFormatting) > 0 {
-		os.Exit(1)
+		exitFunc(1)
 	}
 
 	return nil