feat: v2 dual-write for checkpoints behind feature flag

cmd/entire/cli/checkpoint/v2_store.go

@@ -0,0 +1,88 @@
+package checkpoint
+
+import (
+	"fmt"
+
+	"github.com/go-git/go-git/v6"
+	"github.com/go-git/go-git/v6/plumbing"
+	"github.com/go-git/go-git/v6/plumbing/object"
+)
+
+// V2GitStore provides checkpoint storage operations for the v2 ref layout.
+// It writes to two custom refs under refs/entire/:
+//   - /main: permanent metadata + compact transcripts
+//   - /full/current: active generation of raw transcripts
+//
+// V2GitStore is separate from GitStore (v1) to keep concerns isolated
+// and simplify future v1 removal. It composes GitStore internally to
+// reuse ref-agnostic entry-building helpers (tree surgery, session
+// indexing, summary aggregation).
+type V2GitStore struct {
+	repo *git.Repository
+	gs   *GitStore // shared entry-building helpers (same package)
+}
+
+// NewV2GitStore creates a new v2 checkpoint store backed by the given git repository.
+func NewV2GitStore(repo *git.Repository) *V2GitStore {
+	return &V2GitStore{
+		repo: repo,
+		gs:   &GitStore{repo: repo},
+	}
+}
+
+// ensureRef ensures that a custom ref exists, creating an orphan commit
+// with an empty tree if it does not.
+func (s *V2GitStore) ensureRef(refName plumbing.ReferenceName) error {
+	_, err := s.repo.Reference(refName, true)
+	if err == nil {
+		return nil // Already exists
+	}
+
+	emptyTreeHash, err := BuildTreeFromEntries(s.repo, make(map[string]object.TreeEntry))
+	if err != nil {
+		return fmt.Errorf("failed to build empty tree: %w", err)
+	}
+
+	authorName, authorEmail := GetGitAuthorFromRepo(s.repo)
+	commitHash, err := CreateCommit(s.repo, emptyTreeHash, plumbing.ZeroHash, "Initialize v2 ref", authorName, authorEmail)
+	if err != nil {
+		return fmt.Errorf("failed to create initial commit: %w", err)
+	}
+
+	ref := plumbing.NewHashReference(refName, commitHash)
+	if err := s.repo.Storer.SetReference(ref); err != nil {
+		return fmt.Errorf("failed to set ref %s: %w", refName, err)
+	}
+
+	return nil
+}
+
+// getRefState returns the parent commit hash and root tree hash for a ref.
+func (s *V2GitStore) getRefState(refName plumbing.ReferenceName) (parentHash, treeHash plumbing.Hash, err error) {
+	ref, err := s.repo.Reference(refName, true)
+	if err != nil {
+		return plumbing.ZeroHash, plumbing.ZeroHash, fmt.Errorf("ref %s not found: %w", refName, err)
+	}
+
+	commit, err := s.repo.CommitObject(ref.Hash())
+	if err != nil {
+		return plumbing.ZeroHash, plumbing.ZeroHash, fmt.Errorf("failed to get commit for ref %s: %w", refName, err)
+	}
+
+	return ref.Hash(), commit.TreeHash, nil
+}
+
+// updateRef creates a new commit on a ref with the given tree, updating the ref to point to it.
+func (s *V2GitStore) updateRef(refName plumbing.ReferenceName, treeHash, parentHash plumbing.Hash, message, authorName, authorEmail string) error {
+	commitHash, err := CreateCommit(s.repo, treeHash, parentHash, message, authorName, authorEmail)
+	if err != nil {
+		return fmt.Errorf("failed to create commit: %w", err)
+	}
+
+	ref := plumbing.NewHashReference(refName, commitHash)
+	if err := s.repo.Storer.SetReference(ref); err != nil {
+		return fmt.Errorf("failed to update ref %s: %w", refName, err)
+	}
+
+	return nil
+}