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
+					}
 				}
 			}
 		}

internal/syncbranch/worktree.go

@@ -916,3 +916,25 @@ func HasGitRemote(ctx context.Context) bool {
 	}
 	return len(strings.TrimSpace(string(output))) > 0
 }
+
+// GetCurrentBranch returns the name of the current git branch
+func GetCurrentBranch(ctx context.Context) (string, error) {
+	cmd := exec.CommandContext(ctx, "git", "symbolic-ref", "--short", "HEAD")
+	output, err := cmd.Output()
+	if err != nil {
+		return "", fmt.Errorf("failed to get current branch: %w", err)
+	}
+	return strings.TrimSpace(string(output)), nil
+}
+
+// IsSyncBranchSameAsCurrent returns true if the sync branch is the same as the current branch.
+// This is used to detect the case where we can't use a worktree because the branch is already
+// checked out. In this case, we should commit directly to the current branch instead.
+// See: https://github.com/steveyegge/beads/issues/519
+func IsSyncBranchSameAsCurrent(ctx context.Context, syncBranch string) bool {
+	currentBranch, err := GetCurrentBranch(ctx)
+	if err != nil {
+		return false
+	}
+	return currentBranch == syncBranch
+}

internal/syncbranch/worktree_divergence_test.go

@@ -683,3 +683,70 @@ func TestCountIssuesInContent(t *testing.T) {
 		})
 	}
 }
+
+// TestIsSyncBranchSameAsCurrent tests detection of sync.branch == current branch (GH#519)
+func TestIsSyncBranchSameAsCurrent(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping integration test in short mode")
+	}
+
+	ctx := context.Background()
+
+	t.Run("returns true when sync branch equals current branch", func(t *testing.T) {
+		repoDir := setupTestRepo(t)
+		defer os.RemoveAll(repoDir)
+
+		// Create initial commit so we can get current branch
+		writeFile(t, filepath.Join(repoDir, ".beads", "issues.jsonl"), `{"id":"test-1"}`)
+		runGit(t, repoDir, "add", ".")
+		runGit(t, repoDir, "commit", "-m", "initial commit")
+
+		// Get current branch name
+		currentBranch := strings.TrimSpace(getGitOutput(t, repoDir, "symbolic-ref", "--short", "HEAD"))
+
+		// Save original dir and change to test repo
+		origDir, _ := os.Getwd()
+		os.Chdir(repoDir)
+		defer os.Chdir(origDir)
+
+		// Should return true when sync branch == current branch
+		if !IsSyncBranchSameAsCurrent(ctx, currentBranch) {
+			t.Errorf("IsSyncBranchSameAsCurrent(%q) = false, want true", currentBranch)
+		}
+	})
+
+	t.Run("returns false when sync branch differs from current branch", func(t *testing.T) {
+		repoDir := setupTestRepo(t)
+		defer os.RemoveAll(repoDir)
+
+		// Create initial commit
+		writeFile(t, filepath.Join(repoDir, ".beads", "issues.jsonl"), `{"id":"test-1"}`)
+		runGit(t, repoDir, "add", ".")
+		runGit(t, repoDir, "commit", "-m", "initial commit")
+
+		// Save original dir and change to test repo
+		origDir, _ := os.Getwd()
+		os.Chdir(repoDir)
+		defer os.Chdir(origDir)
+
+		// Should return false when sync branch != current branch
+		if IsSyncBranchSameAsCurrent(ctx, "beads-sync") {
+			t.Error("IsSyncBranchSameAsCurrent(\"beads-sync\") = true, want false")
+		}
+	})
+
+	t.Run("returns false on error getting current branch", func(t *testing.T) {
+		// Test in a non-git directory
+		tmpDir, _ := os.MkdirTemp("", "non-git-*")
+		defer os.RemoveAll(tmpDir)
+
+		origDir, _ := os.Getwd()
+		os.Chdir(tmpDir)
+		defer os.Chdir(origDir)
+
+		// Should return false when not in a git repo
+		if IsSyncBranchSameAsCurrent(ctx, "any-branch") {
+			t.Error("IsSyncBranchSameAsCurrent in non-git dir = true, want false")
+		}
+	})
+}