test: add coverage for GH#807 sync.branch main/master rejection

- Add TestSet cases for main/master rejection in syncbranch_test.go
- Add TestInitWithSyncBranch to verify --branch flag works
- Add TestInitWithoutBranchFlag to verify no auto-detection (root cause)

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

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

cmd/bd/init_test.go

@@ -183,6 +183,96 @@ func TestInitCommand(t *testing.T) {
 
 // Note: Error case testing is omitted because the init command calls os.Exit()
 // on errors, which makes it difficult to test in a unit test context.
+// GH#807: Rejection of main/master as sync branch is tested at unit level in
+// internal/syncbranch/syncbranch_test.go (TestValidateSyncBranchName, TestSet).
+
+// TestInitWithSyncBranch verifies that --branch flag correctly sets sync.branch
+// GH#807: Also verifies that valid sync branches work (rejection is tested at unit level)
+func TestInitWithSyncBranch(t *testing.T) {
+	// Reset global state
+	origDBPath := dbPath
+	defer func() { dbPath = origDBPath }()
+	dbPath = ""
+
+	// Reset Cobra flags
+	initCmd.Flags().Set("branch", "")
+
+	tmpDir := t.TempDir()
+	t.Chdir(tmpDir)
+
+	// Initialize git repo first (needed for sync branch to make sense)
+	if err := runCommandInDir(tmpDir, "git", "init", "--initial-branch=dev"); err != nil {
+		t.Fatalf("Failed to init git: %v", err)
+	}
+
+	// Run bd init with --branch flag
+	rootCmd.SetArgs([]string{"init", "--prefix", "test", "--branch", "beads-sync", "--quiet"})
+	if err := rootCmd.Execute(); err != nil {
+		t.Fatalf("Init with --branch failed: %v", err)
+	}
+
+	// Verify database was created
+	dbFilePath := filepath.Join(tmpDir, ".beads", "beads.db")
+	store, err := openExistingTestDB(t, dbFilePath)
+	if err != nil {
+		t.Fatalf("Failed to open database: %v", err)
+	}
+	defer store.Close()
+
+	// Verify sync.branch was set correctly
+	ctx := context.Background()
+	syncBranch, err := store.GetConfig(ctx, "sync.branch")
+	if err != nil {
+		t.Fatalf("Failed to get sync.branch from database: %v", err)
+	}
+	if syncBranch != "beads-sync" {
+		t.Errorf("Expected sync.branch 'beads-sync', got %q", syncBranch)
+	}
+}
+
+// TestInitWithoutBranchFlag verifies that sync.branch is NOT auto-set when --branch is omitted
+// GH#807: This was the root cause - init was auto-detecting current branch (e.g., main)
+func TestInitWithoutBranchFlag(t *testing.T) {
+	// Reset global state
+	origDBPath := dbPath
+	defer func() { dbPath = origDBPath }()
+	dbPath = ""
+
+	// Reset Cobra flags
+	initCmd.Flags().Set("branch", "")
+
+	tmpDir := t.TempDir()
+	t.Chdir(tmpDir)
+
+	// Initialize git repo on 'main' branch
+	if err := runCommandInDir(tmpDir, "git", "init", "--initial-branch=main"); err != nil {
+		t.Fatalf("Failed to init git: %v", err)
+	}
+
+	// Run bd init WITHOUT --branch flag
+	rootCmd.SetArgs([]string{"init", "--prefix", "test", "--quiet"})
+	if err := rootCmd.Execute(); err != nil {
+		t.Fatalf("Init failed: %v", err)
+	}
+
+	// Verify database was created
+	dbFilePath := filepath.Join(tmpDir, ".beads", "beads.db")
+	store, err := openExistingTestDB(t, dbFilePath)
+	if err != nil {
+		t.Fatalf("Failed to open database: %v", err)
+	}
+	defer store.Close()
+
+	// Verify sync.branch was NOT set (empty = use current branch directly)
+	ctx := context.Background()
+	syncBranch, err := store.GetConfig(ctx, "sync.branch")
+	if err != nil {
+		t.Fatalf("Failed to get sync.branch from database: %v", err)
+	}
+	if syncBranch != "" {
+		t.Errorf("Expected sync.branch to be empty (not auto-detected), got %q", syncBranch)
+	}
+}
 
 func TestInitAlreadyInitialized(t *testing.T) {
 	// Reset global state