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>

cmd/bd/branch.go

@@ -0,0 +1,85 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+	"github.com/steveyegge/beads/internal/storage"
+	"github.com/steveyegge/beads/internal/ui"
+)
+
+var branchCmd = &cobra.Command{
+	Use:     "branch [name]",
+	GroupID: "sync",
+	Short:   "List or create branches (requires Dolt backend)",
+	Long: `List all branches or create a new branch.
+
+This command requires the Dolt storage backend. Without arguments,
+it lists all branches. With an argument, it creates a new branch.
+
+Examples:
+  bd branch                    # List all branches
+  bd branch feature-xyz        # Create a new branch named feature-xyz`,
+	Args: cobra.MaximumNArgs(1),
+	Run: func(cmd *cobra.Command, args []string) {
+		ctx := rootCtx
+
+		// Check if storage supports versioning
+		vs, ok := storage.AsVersioned(store)
+		if !ok {
+			FatalErrorRespectJSON("branch requires Dolt backend (current backend does not support versioning)")
+		}
+
+		// If no args, list branches
+		if len(args) == 0 {
+			branches, err := vs.ListBranches(ctx)
+			if err != nil {
+				FatalErrorRespectJSON("failed to list branches: %v", err)
+			}
+
+			currentBranch, err := vs.CurrentBranch(ctx)
+			if err != nil {
+				// Non-fatal, just don't show current marker
+				currentBranch = ""
+			}
+
+			if jsonOutput {
+				outputJSON(map[string]interface{}{
+					"current":  currentBranch,
+					"branches": branches,
+				})
+				return
+			}
+
+			fmt.Printf("\n%s Branches:\n\n", ui.RenderAccent("🌿"))
+			for _, branch := range branches {
+				if branch == currentBranch {
+					fmt.Printf("  * %s\n", ui.StatusInProgressStyle.Render(branch))
+				} else {
+					fmt.Printf("    %s\n", branch)
+				}
+			}
+			fmt.Println()
+			return
+		}
+
+		// Create new branch
+		branchName := args[0]
+		if err := vs.Branch(ctx, branchName); err != nil {
+			FatalErrorRespectJSON("failed to create branch: %v", err)
+		}
+
+		if jsonOutput {
+			outputJSON(map[string]interface{}{
+				"created": branchName,
+			})
+			return
+		}
+
+		fmt.Printf("Created branch: %s\n", ui.RenderAccent(branchName))
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(branchCmd)
+}