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/history.go

@@ -288,8 +288,9 @@ type IssueDiff struct {
 	ToDescription   string
 }
 
-// GetConflicts returns any merge conflicts in the current state
-func (s *DoltStore) GetConflicts(ctx context.Context) ([]*Conflict, error) {
+// GetInternalConflicts returns any merge conflicts in the current state (internal format).
+// For the public interface, use GetConflicts which returns storage.Conflict.
+func (s *DoltStore) GetInternalConflicts(ctx context.Context) ([]*TableConflict, error) {
 	rows, err := s.db.QueryContext(ctx, `
 		SELECT table_name, num_conflicts FROM dolt_conflicts
 	`)
@@ -298,9 +299,9 @@ func (s *DoltStore) GetConflicts(ctx context.Context) ([]*Conflict, error) {
 	}
 	defer rows.Close()
 
-	var conflicts []*Conflict
+	var conflicts []*TableConflict
 	for rows.Next() {
-		var c Conflict
+		var c TableConflict
 		if err := rows.Scan(&c.TableName, &c.NumConflicts); err != nil {
 			return nil, fmt.Errorf("failed to scan conflict: %w", err)
 		}
@@ -310,8 +311,8 @@ func (s *DoltStore) GetConflicts(ctx context.Context) ([]*Conflict, error) {
 	return conflicts, rows.Err()
 }
 
-// Conflict represents a merge conflict
-type Conflict struct {
+// TableConflict represents a Dolt table-level merge conflict (internal representation).
+type TableConflict struct {
 	TableName    string
 	NumConflicts int
 }