feat(storage): add VersionedStorage interface with history/diff/branch operations

Extends Storage interface with Dolt-specific version control capabilities:

- New VersionedStorage interface in storage/versioned.go with:
  - History queries: History(), AsOf(), Diff()
  - Branch operations: Branch(), Merge(), CurrentBranch(), ListBranches()
  - Commit operations: Commit(), GetCurrentCommit()
  - Conflict resolution: GetConflicts(), ResolveConflicts()
  - Helper types: HistoryEntry, DiffEntry, Conflict

- DoltStore implements VersionedStorage interface

- New CLI commands:
  - bd history <id> - Show issue version history
  - bd diff <from> <to> - Show changes between commits/branches
  - bd branch [name] - List or create branches
  - bd vc merge <branch> - Merge branch to current
  - bd vc commit -m <msg> - Create a commit
  - bd vc status - Show current branch/commit

- Added --as-of flag to bd show for time-travel queries

- IsVersioned() helper for graceful SQLite backend detection

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/dolt/store.go

@@ -28,6 +28,8 @@ import (
 
 	// Import Dolt driver
 	_ "github.com/dolthub/driver"
+
+	"github.com/steveyegge/beads/internal/storage"
 )
 
 // DoltStore implements the Storage interface using Dolt
@@ -342,13 +344,19 @@ func (s *DoltStore) Checkout(ctx context.Context, branch string) error {
 	return nil
 }
 
-// Merge merges the specified branch into the current branch
-func (s *DoltStore) Merge(ctx context.Context, branch string) error {
+// Merge merges the specified branch into the current branch.
+// Returns any merge conflicts if present. Implements storage.VersionedStorage.
+func (s *DoltStore) Merge(ctx context.Context, branch string) ([]storage.Conflict, error) {
 	_, err := s.db.ExecContext(ctx, "CALL DOLT_MERGE(?)", branch)
 	if err != nil {
-		return fmt.Errorf("failed to merge branch %s: %w", branch, err)
+		// Check if the error is due to conflicts
+		conflicts, conflictErr := s.GetConflicts(ctx)
+		if conflictErr == nil && len(conflicts) > 0 {
+			return conflicts, nil
+		}
+		return nil, fmt.Errorf("failed to merge branch %s: %w", branch, err)
 	}
-	return nil
+	return nil, nil
 }
 
 // CurrentBranch returns the current branch name