feat: add replace subcommand with multi-pattern support

internal/replace/request.go

@@ -0,0 +1,61 @@
+package replace
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+// ReplaceRequest captures all inputs needed to evaluate a replace operation.
+type ReplaceRequest struct {
+	WorkingDir         string
+	Patterns           []string
+	Replacement        string
+	IncludeDirectories bool
+	Recursive          bool
+	IncludeHidden      bool
+	Extensions         []string
+}
+
+// Validate ensures the request is well-formed before preview/apply.
+func (r *ReplaceRequest) Validate() error {
+	if r == nil {
+		return errors.New("replace request cannot be nil")
+	}
+
+	if len(r.Patterns) == 0 {
+		return errors.New("at least one pattern is required")
+	}
+
+	if r.Replacement == "" {
+		// Allow empty replacement but make sure caller has surfaced warnings elsewhere.
+		// No error returned; preview will message accordingly.
+	}
+
+	if r.WorkingDir == "" {
+		cwd, err := os.Getwd()
+		if err != nil {
+			return fmt.Errorf("determine working directory: %w", err)
+		}
+		r.WorkingDir = cwd
+	}
+
+	if !filepath.IsAbs(r.WorkingDir) {
+		abs, err := filepath.Abs(r.WorkingDir)
+		if err != nil {
+			return fmt.Errorf("resolve working directory: %w", err)
+		}
+		r.WorkingDir = abs
+	}
+
+	info, err := os.Stat(r.WorkingDir)
+	if err != nil {
+		return fmt.Errorf("stat working directory: %w", err)
+	}
+	if !info.IsDir() {
+		return fmt.Errorf("working directory %q is not a directory", r.WorkingDir)
+	}
+
+	return nil
+}