fix(dashboard): don't mark agent inbox messages read when user views them

When the dashboard user browses another agent's mailbox (e.g. the CEO's
inbox), opening a message no longer triggers POST /messages/:id/read.
The previous behavior silently consumed the agent's unread state, so the
agent's heartbeat never surfaced the message and fn_read_messages (which
defaults to unread_only=true) returned nothing. Auto-mark-read now only
fires on the dashboard user's own inbox tab. Adds regression tests in
MailboxView.test.tsx and MailboxModal.test.tsx.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/app/components/MailboxModal.tsx

@@ -207,8 +207,10 @@ export function MailboxModal({
 
   const handleOpenMessage = useCallback(async (message: Message) => {
     setSelectedMessage(message);
-    // Mark as read if unread
-    if (!message.read) {
+    // Only auto-mark as read when viewing the dashboard user's own inbox.
+    // Browsing another agent's mailbox must not consume their unread messages
+    // out from under them — the agent's heartbeat is the one that reads + acks.
+    if (!message.read && activeTab === "inbox") {
       try {
         const updated = await markMessageRead(message.id, projectId);
         // Update inbox state
@@ -233,7 +235,7 @@ export function MailboxModal({
     } catch {
       setConversationMessages([message]);
     }
-  }, [projectId]);
+  }, [projectId, activeTab]);
 
   const handleCloseMessage = useCallback(() => {
     setSelectedMessage(null);