fix: prevent sync.branch from being set to main/master (#807)

Two issues fixed:
1. `bd init` was auto-detecting current branch (e.g., main) as sync.branch
   when no --branch flag was specified. This caused worktree conflicts.
2. Added validation to reject main/master as sync.branch values.

When sync.branch is set to main, the worktree mechanism creates a checkout
of main at .git/beads-worktrees/main/, which prevents git checkout main
from working in the user's working directory.

The sync branch feature should use a dedicated branch like 'beads-sync'.

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

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

Executed-By: beads/crew/dave
Rig: beads
Role: crew

internal/syncbranch/syncbranch.go

@@ -65,6 +65,23 @@ func ValidateBranchName(name string) error {
 	return nil
 }
 
+// ValidateSyncBranchName checks if a branch name is valid for use as sync.branch.
+// GH#807: Setting sync.branch to 'main' or 'master' causes problems because the
+// worktree mechanism will check out that branch, preventing the user from checking
+// it out in their working directory.
+func ValidateSyncBranchName(name string) error {
+	if err := ValidateBranchName(name); err != nil {
+		return err
+	}
+
+	// GH#807: Reject main/master as sync branch - these cause worktree conflicts
+	if name == "main" || name == "master" {
+		return fmt.Errorf("cannot use '%s' as sync branch: git worktrees prevent checking out the same branch in multiple locations. Use a dedicated branch like 'beads-sync' instead", name)
+	}
+
+	return nil
+}
+
 // Get retrieves the sync branch configuration with the following precedence:
 // 1. BEADS_SYNC_BRANCH environment variable
 // 2. sync-branch from config.yaml (version controlled, shared across clones)
@@ -178,7 +195,8 @@ func getConfigFromDB(dbPath string, key string) string {
 
 // Set stores the sync branch configuration in the database
 func Set(ctx context.Context, store storage.Storage, branch string) error {
-	if err := ValidateBranchName(branch); err != nil {
+	// GH#807: Use sync-specific validation that rejects main/master
+	if err := ValidateSyncBranchName(branch); err != nil {
 		return err
 	}
 

internal/syncbranch/syncbranch_test.go

@@ -48,6 +48,36 @@ func TestValidateBranchName(t *testing.T) {
 	}
 }
 
+func TestValidateSyncBranchName(t *testing.T) {
+	tests := []struct {
+		name    string
+		branch  string
+		wantErr bool
+	}{
+		// Valid sync branches
+		{"beads-sync is valid", "beads-sync", false},
+		{"feature branch is valid", "feature-branch", false},
+		{"empty is valid", "", false},
+
+		// GH#807: main and master should be rejected for sync branch
+		{"main is invalid for sync", "main", true},
+		{"master is invalid for sync", "master", true},
+
+		// Standard branch name validation still applies
+		{"invalid: HEAD", "HEAD", true},
+		{"invalid: contains ..", "feature..branch", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := ValidateSyncBranchName(tt.branch)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ValidateSyncBranchName(%q) error = %v, wantErr %v", tt.branch, err, tt.wantErr)
+			}
+		})
+	}
+}
+
 func newTestStore(t *testing.T) *sqlite.SQLiteStorage {
 	t.Helper()
 	store, err := sqlite.New(context.Background(), "file::memory:?mode=memory&cache=private")