fix(setup): avoid null values in Claude settings hooks (GH#955)

When removing all hooks from an event, the key was being set to null
instead of being deleted. Claude Code expects arrays, not null values,
causing startup failures with 'Expected array, but received null'.

Changes:
- removeHookCommand now deletes the event key when no hooks remain
- installClaude cleans up any existing null values from buggy removal
- Added tests for null value prevention and cleanup

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

cmd/bd/setup/claude_test.go

@@ -281,6 +281,94 @@ func TestRemoveHookCommand(t *testing.T) {
 	}
 }
 
+// TestRemoveHookCommandNoNull verifies that removing all hooks deletes the key
+// instead of setting it to null. GH#955: null values in hooks cause Claude Code to fail.
+func TestRemoveHookCommandNoNull(t *testing.T) {
+	hooks := map[string]interface{}{
+		"SessionStart": []interface{}{
+			map[string]interface{}{
+				"matcher": "",
+				"hooks": []interface{}{
+					map[string]interface{}{
+						"type":    "command",
+						"command": "bd prime",
+					},
+				},
+			},
+		},
+	}
+
+	removeHookCommand(hooks, "SessionStart", "bd prime")
+
+	// Key should be deleted, not set to null or empty array
+	if _, exists := hooks["SessionStart"]; exists {
+		t.Error("Expected SessionStart key to be deleted after removing all hooks")
+	}
+
+	// Verify JSON serialization doesn't produce null
+	data, err := json.Marshal(hooks)
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	if strings.Contains(string(data), "null") {
+		t.Errorf("JSON contains null: %s", data)
+	}
+}
+
+// TestInstallClaudeCleanupNullHooks verifies that install cleans up existing null values.
+// GH#955: null values left by previous buggy removal cause Claude Code to fail.
+func TestInstallClaudeCleanupNullHooks(t *testing.T) {
+	env, stdout, _ := newClaudeTestEnv(t)
+
+	// Create settings file with null hooks (simulating the bug)
+	settingsPath := globalSettingsPath(env.homeDir)
+	writeSettings(t, settingsPath, map[string]interface{}{
+		"hooks": map[string]interface{}{
+			"SessionStart": nil,
+			"PreCompact":   nil,
+		},
+	})
+
+	// Install should clean up null values and add proper hooks
+	err := installClaude(env, false, false)
+	if err != nil {
+		t.Fatalf("install failed: %v", err)
+	}
+
+	// Verify hooks were properly added
+	if !strings.Contains(stdout.String(), "Registered SessionStart hook") {
+		t.Error("Expected SessionStart hook to be registered")
+	}
+
+	// Read back the file and verify no null values
+	data, err := env.readFile(settingsPath)
+	if err != nil {
+		t.Fatalf("read settings: %v", err)
+	}
+	if strings.Contains(string(data), "null") {
+		t.Errorf("Settings file still contains null: %s", data)
+	}
+
+	// Verify it parses as valid Claude settings
+	var settings map[string]interface{}
+	if err := json.Unmarshal(data, &settings); err != nil {
+		t.Fatalf("parse settings: %v", err)
+	}
+	hooks, ok := settings["hooks"].(map[string]interface{})
+	if !ok {
+		t.Fatal("hooks section missing")
+	}
+	for _, event := range []string{"SessionStart", "PreCompact"} {
+		eventHooks, ok := hooks[event].([]interface{})
+		if !ok {
+			t.Errorf("%s should be an array, not nil or missing", event)
+		}
+		if len(eventHooks) == 0 {
+			t.Errorf("%s should have hooks", event)
+		}
+	}
+}
+
 func TestHasBeadsHooks(t *testing.T) {
 	tmpDir := t.TempDir()