feat(rename): add bd rename command for renaming issue IDs

Add new `bd rename <old-id> <new-id>` command that:
- Updates the issue's primary ID
- Updates text references in other issues (title, description, etc.)
- Handles dependencies via storage layer's UpdateIssueID

Also fix sync.go build error where hasSyncBranchConfig/syncBranchName
were referenced but not defined - should use sbc.IsConfigured()/sbc.Branch.

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

cmd/bd/sync.go

@@ -344,11 +344,11 @@ The --full flag provides the legacy full sync behavior for backwards compatibili
 		// GH#1166: Block sync if currently on the sync branch
 		// This must happen BEFORE worktree operations - after entering a worktree,
 		// GetCurrentBranch() would return the worktree's branch, not the original.
-		if hasSyncBranchConfig {
-			if syncbranch.IsSyncBranchSameAsCurrent(ctx, syncBranchName) {
+		if sbc.IsConfigured() {
+			if syncbranch.IsSyncBranchSameAsCurrent(ctx, sbc.Branch) {
 				FatalError("Cannot sync to '%s': it's your current branch. "+
 					"Checkout a different branch first, or use a dedicated sync branch like 'beads-sync'.",
-					syncBranchName)
+					sbc.Branch)
 			}
 		}