fix(sync): handle sync.branch == current branch (GH#519)

When sync.branch is configured to the same branch as the current branch,
git worktree creation fails because the same branch cannot be checked out
in multiple locations.

This fix detects when sync.branch equals the current branch and falls back
to direct commits on the current branch instead of using the worktree-based
approach.

Changes:
- Add IsSyncBranchSameAsCurrent() helper in syncbranch package
- Add GetCurrentBranch() helper function
- Update sync.go to detect this case and skip worktree operations
- Add unit tests for the new functionality

Closes #519

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/sync.go

@@ -380,12 +380,20 @@ Use --merge to merge the sync branch back to main branch.`,
 		if err := ensureStoreActive(); err == nil && store != nil {
 			syncBranchName, _ = syncbranch.Get(ctx, store)
 			if syncBranchName != "" && syncbranch.HasGitRemote(ctx) {
-				repoRoot, err = syncbranch.GetRepoRoot(ctx)
-				if err != nil {
-					fmt.Fprintf(os.Stderr, "Warning: sync.branch configured but failed to get repo root: %v\n", err)
-					fmt.Fprintf(os.Stderr, "Falling back to current branch commits\n")
+				// GH#519: Check if sync.branch equals current branch
+				// If so, we can't use a worktree (git doesn't allow same branch in multiple worktrees)
+				// Fall back to direct commits on the current branch
+				if syncbranch.IsSyncBranchSameAsCurrent(ctx, syncBranchName) {
+					// sync.branch == current branch - use regular commits, not worktree
+					useSyncBranch = false
 				} else {
-					useSyncBranch = true
+					repoRoot, err = syncbranch.GetRepoRoot(ctx)
+					if err != nil {
+						fmt.Fprintf(os.Stderr, "Warning: sync.branch configured but failed to get repo root: %v\n", err)
+						fmt.Fprintf(os.Stderr, "Falling back to current branch commits\n")
+					} else {
+						useSyncBranch = true
+					}
 				}
 			}
 		}