Checkpoints V2: add migration option

cmd/entire/cli/checkpoint/committed.go

@@ -350,7 +350,7 @@ func (s *GitStore) writeSessionToSubdirectory(ctx context.Context, opts WriteCom
 
 	// Write prompts
 	if len(opts.Prompts) > 0 {
-		promptContent := redact.String(strings.Join(opts.Prompts, "\n\n---\n\n"))
+		promptContent := redact.String(JoinPrompts(opts.Prompts))
 		blobHash, err := CreateBlobFromContent(s.repo, []byte(promptContent))
 		if err != nil {
 			return filePaths, err
@@ -1281,7 +1281,7 @@ func (s *GitStore) UpdateCommitted(ctx context.Context, opts UpdateCommittedOpti
 
 	// Replace prompts (apply redaction as safety net)
 	if len(opts.Prompts) > 0 {
-		promptContent := redact.String(strings.Join(opts.Prompts, "\n\n---\n\n"))
+		promptContent := redact.String(JoinPrompts(opts.Prompts))
 		blobHash, err := CreateBlobFromContent(s.repo, []byte(promptContent))
 		if err != nil {
 			return fmt.Errorf("failed to create prompt blob: %w", err)

cmd/entire/cli/checkpoint/prompts.go

@@ -0,0 +1,25 @@
+package checkpoint
+
+import "strings"
+
+// PromptSeparator is the canonical separator used in prompt.txt when multiple
+// prompts are stored in a single file.
+const PromptSeparator = "\n\n---\n\n"
+
+// JoinPrompts serializes prompts to prompt.txt format.
+func JoinPrompts(prompts []string) string {
+	return strings.Join(prompts, PromptSeparator)
+}
+
+// SplitPromptContent deserializes prompt.txt content into individual prompts.
+func SplitPromptContent(content string) []string {
+	if content == "" {
+		return nil
+	}
+
+	prompts := strings.Split(content, PromptSeparator)
+	for len(prompts) > 0 && prompts[len(prompts)-1] == "" {
+		prompts = prompts[:len(prompts)-1]
+	}
+	return prompts
+}

cmd/entire/cli/checkpoint/prompts_test.go

@@ -0,0 +1,29 @@
+package checkpoint
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestJoinAndSplitPrompts_RoundTrip(t *testing.T) {
+	t.Parallel()
+
+	original := []string{
+		"first line\nwith newline",
+		"second prompt",
+	}
+
+	joined := JoinPrompts(original)
+	split := SplitPromptContent(joined)
+
+	require.Len(t, split, 2)
+	assert.Equal(t, original, split)
+}
+
+func TestSplitPromptContent_EmptyContent(t *testing.T) {
+	t.Parallel()
+
+	assert.Nil(t, SplitPromptContent(""))
+}

cmd/entire/cli/checkpoint/v2_committed.go

@@ -121,7 +121,7 @@ func (s *V2GitStore) updateCommittedMain(ctx context.Context, opts UpdateCommitt
 	sessionPath := fmt.Sprintf("%s%d/", basePath, sessionIndex)
 
 	if len(opts.Prompts) > 0 {
-		promptContent := redact.String(strings.Join(opts.Prompts, "\n\n---\n\n"))
+		promptContent := redact.String(JoinPrompts(opts.Prompts))
 		blobHash, err := CreateBlobFromContent(s.repo, []byte(promptContent))
 		if err != nil {
 			return 0, fmt.Errorf("failed to create prompt blob: %w", err)
@@ -334,7 +334,7 @@ func (s *V2GitStore) writeMainSessionToSubdirectory(opts WriteCommittedOptions,
 
 	// Write prompts
 	if len(opts.Prompts) > 0 {
-		promptContent := redact.String(strings.Join(opts.Prompts, "\n\n---\n\n"))
+		promptContent := redact.String(JoinPrompts(opts.Prompts))
 		blobHash, err := CreateBlobFromContent(s.repo, []byte(promptContent))
 		if err != nil {
 			return filePaths, err