close worktree transport and lifecycle contract gaps

internal/continuity/continuity.go

@@ -128,6 +128,22 @@ func ValidatePickup(root string, h Handoff, taskFileSHA string) error {
 	return nil
 }
 
+// VerifyTaskFile ensures the worktree contains the original, immutable task.
+func VerifyTaskFile(root, taskFileSHA string) error {
+	if taskFileSHA == "" {
+		return errors.New("TASK.md hash required")
+	}
+	b, err := os.ReadFile(filepath.Join(root, "TASK.md"))
+	if err != nil {
+		return err
+	}
+	sum := sha256.Sum256(b)
+	if hex.EncodeToString(sum[:]) != taskFileSHA {
+		return errors.New("TASK.md changed")
+	}
+	return nil
+}
+
 type CAS interface {
 	PutArtifact([]byte) (string, error)
 	Artifact(string) ([]byte, error)
@@ -175,6 +191,9 @@ func ScratchCommit(root, branch, message string) error {
 	if branch == "" || strings.ContainsAny(branch, " \t\n") {
 		return errors.New("invalid scratch branch")
 	}
+	if strings.TrimSpace(message) == "" {
+		return errors.New("scratch commit message required")
+	}
 	for _, args := range [][]string{{"switch", "-c", branch}, {"add", "-A"}, {"commit", "-m", message}} {
 		if err := exec.Command("git", append([]string{"-C", root}, args...)...).Run(); err != nil {
 			return err
@@ -182,3 +201,22 @@ func ScratchCommit(root, branch, message string) error {
 	}
 	return nil
 }
+
+// ScratchSync pushes/pulls a scratch branch. Pull uses fast-forward-only to
+// avoid silently merging independent WIP histories.
+func ScratchSync(root, branch, remote string, push bool) error {
+	if branch == "" || strings.ContainsAny(branch, " \t\n") || remote == "" {
+		return errors.New("invalid scratch sync")
+	}
+	args := []string{"-C", root, "push", remote, branch}
+	if !push {
+		args = []string{"-C", root, "fetch", remote, branch}
+	}
+	if err := exec.Command("git", args...).Run(); err != nil {
+		return err
+	}
+	if !push {
+		return exec.Command("git", "-C", root, "merge", "--ff-only", "FETCH_HEAD").Run()
+	}
+	return nil
+}