feat(v0.3): add CopySeedDir with task.yaml/.ctask skip

CopySeedDir is the v0.3 user seed overlay primitive: recursive copy
from a seed directory to a workspace, overwriting destination
files. task.yaml and the .ctask metadata directory at the seed root
are intentionally skipped so a stale or hostile seed cannot
overwrite ctask-owned state. Missing src is a no-op (the seed
directory is optional).

internal/seed/copy.go

@@ -0,0 +1,93 @@
+package seed
+
+import (
+	"errors"
+	"io"
+	"os"
+	"path/filepath"
+)
+
+// CopySeedDir recursively copies the contents of srcDir into dstDir.
+// Files at the destination are overwritten. Subdirectories are preserved.
+//
+// The following entries at the top level of srcDir are intentionally skipped:
+//   - task.yaml (ctask owns this file)
+//   - .ctask    (directory; reserved for ctask metadata)
+//
+// If srcDir does not exist, this function returns nil (no-op). Other I/O errors
+// are returned to the caller.
+func CopySeedDir(srcDir, dstDir string) error {
+	info, err := os.Stat(srcDir)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return nil
+		}
+		return err
+	}
+	if !info.IsDir() {
+		return nil
+	}
+
+	entries, err := os.ReadDir(srcDir)
+	if err != nil {
+		return err
+	}
+
+	for _, entry := range entries {
+		name := entry.Name()
+		if name == "task.yaml" || name == ".ctask" {
+			continue
+		}
+		srcPath := filepath.Join(srcDir, name)
+		dstPath := filepath.Join(dstDir, name)
+		if err := copyEntry(srcPath, dstPath); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func copyEntry(src, dst string) error {
+	info, err := os.Stat(src)
+	if err != nil {
+		return err
+	}
+	if info.IsDir() {
+		if err := os.MkdirAll(dst, 0755); err != nil {
+			return err
+		}
+		entries, err := os.ReadDir(src)
+		if err != nil {
+			return err
+		}
+		for _, e := range entries {
+			if err := copyEntry(filepath.Join(src, e.Name()), filepath.Join(dst, e.Name())); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+	return copyFile(src, dst, info.Mode())
+}
+
+func copyFile(src, dst string, mode os.FileMode) error {
+	in, err := os.Open(src)
+	if err != nil {
+		return err
+	}
+	defer in.Close()
+
+	if err := os.MkdirAll(filepath.Dir(dst), 0755); err != nil {
+		return err
+	}
+	out, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, mode.Perm())
+	if err != nil {
+		return err
+	}
+	defer out.Close()
+
+	if _, err := io.Copy(out, in); err != nil {
+		return err
+	}
+	return nil
+}