feat(FN-3585): document post-install plugin capabilities and fix mailbox sp

The merge adds a test isolation script (`scripts/check-test-isolation.mjs`) to filter runtime noise from live Fusion app paths, fixes MailboxModal spacing by replacing hardcoded values with design tokens (FN-3585), and documents post-install plugin capabilities in the plugin management guide (FN-358

Fusion-Task-Id: FN-3585

packages/dashboard/app/components/MailboxModal.css

@@ -880,13 +880,13 @@
 }
 
 .message-composer-field--wake {
-  margin-top: 4px;
+  margin-top: var(--space-xs);
 }
 
 .message-composer-wake-label {
   display: flex;
   align-items: center;
-  gap: 8px;
+  gap: var(--space-sm);
   font-size: 0.85rem;
   color: var(--text-secondary, var(--text-muted));
   cursor: pointer;
@@ -897,7 +897,7 @@
 }
 
 .message-composer-wake-hint {
-  margin-left: 6px;
+  margin-left: var(--space-xs);
   font-size: 0.75rem;
   color: var(--text-muted);
 }