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
+}

internal/seed/copy_test.go

@@ -0,0 +1,107 @@
+package seed
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func writeFile(t *testing.T, path, content string) {
+	t.Helper()
+	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		t.Fatalf("mkdir %s: %v", filepath.Dir(path), err)
+	}
+	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+		t.Fatalf("write %s: %v", path, err)
+	}
+}
+
+func readFile(t *testing.T, path string) string {
+	t.Helper()
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read %s: %v", path, err)
+	}
+	return string(data)
+}
+
+func TestCopySeedDirCopiesFilesAndSubdirs(t *testing.T) {
+	src := t.TempDir()
+	dst := t.TempDir()
+
+	writeFile(t, filepath.Join(src, "CLAUDE.md"), "user claude")
+	writeFile(t, filepath.Join(src, "notes.md"), "user notes")
+	writeFile(t, filepath.Join(src, "docs", "intro.md"), "doc body")
+	writeFile(t, filepath.Join(src, "reference", "links.md"), "links")
+
+	if err := CopySeedDir(src, dst); err != nil {
+		t.Fatalf("CopySeedDir: %v", err)
+	}
+
+	if got := readFile(t, filepath.Join(dst, "CLAUDE.md")); got != "user claude" {
+		t.Errorf("CLAUDE.md content: got %q", got)
+	}
+	if got := readFile(t, filepath.Join(dst, "docs", "intro.md")); got != "doc body" {
+		t.Errorf("docs/intro.md content: got %q", got)
+	}
+	if got := readFile(t, filepath.Join(dst, "reference", "links.md")); got != "links" {
+		t.Errorf("reference/links.md content: got %q", got)
+	}
+}
+
+func TestCopySeedDirOverwritesExistingFiles(t *testing.T) {
+	src := t.TempDir()
+	dst := t.TempDir()
+
+	writeFile(t, filepath.Join(dst, "CLAUDE.md"), "builtin")
+	writeFile(t, filepath.Join(src, "CLAUDE.md"), "user")
+
+	if err := CopySeedDir(src, dst); err != nil {
+		t.Fatalf("CopySeedDir: %v", err)
+	}
+	if got := readFile(t, filepath.Join(dst, "CLAUDE.md")); got != "user" {
+		t.Errorf("expected user seed to overwrite, got %q", got)
+	}
+}
+
+func TestCopySeedDirSkipsTaskYAML(t *testing.T) {
+	src := t.TempDir()
+	dst := t.TempDir()
+
+	writeFile(t, filepath.Join(src, "task.yaml"), "id: bogus")
+	writeFile(t, filepath.Join(dst, "task.yaml"), "id: real")
+
+	if err := CopySeedDir(src, dst); err != nil {
+		t.Fatalf("CopySeedDir: %v", err)
+	}
+	if got := readFile(t, filepath.Join(dst, "task.yaml")); got != "id: real" {
+		t.Errorf("task.yaml should not be overwritten by seed: got %q", got)
+	}
+}
+
+func TestCopySeedDirSkipsCtaskDir(t *testing.T) {
+	src := t.TempDir()
+	dst := t.TempDir()
+
+	writeFile(t, filepath.Join(src, ".ctask", "manifest-start.json"), "{}")
+	writeFile(t, filepath.Join(src, "CLAUDE.md"), "ok")
+
+	if err := CopySeedDir(src, dst); err != nil {
+		t.Fatalf("CopySeedDir: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(dst, ".ctask")); !os.IsNotExist(err) {
+		t.Errorf(".ctask should be skipped, but it exists in dst")
+	}
+	if got := readFile(t, filepath.Join(dst, "CLAUDE.md")); got != "ok" {
+		t.Errorf("CLAUDE.md content: got %q", got)
+	}
+}
+
+func TestCopySeedDirMissingSrcIsNoOp(t *testing.T) {
+	dst := t.TempDir()
+	missing := filepath.Join(t.TempDir(), "does-not-exist")
+
+	if err := CopySeedDir(missing, dst); err != nil {
+		t.Errorf("missing seed src should be a no-op, got error: %v", err)
+	}
+}