docs: fix wrong defaults and config path in FAQ

- session.idleMinutes default: 60 -> 0 (disabled by default, per DEFAULT_IDLE_MINUTES)
- crossContext config path: agents.defaults.tools.message.crossContext -> tools.message.crossContext (matches schema)
- Remove incorrect per-agent tools.message override claim

docs/help/faq.md

@@ -1781,9 +1781,10 @@ for usage/billing and raise limits as needed.
   </Accordion>
 
   <Accordion title="Do sessions reset automatically if I never send /new?">
-    Yes. Sessions expire after `session.idleMinutes` (default **60**). The **next**
-    message starts a fresh session id for that chat key. This does not delete
-    transcripts - it just starts a new session.
+    Sessions can expire after `session.idleMinutes`, but this is **disabled by default** (default **0**).
+    Set it to a positive value to enable idle expiry. When enabled, the **next**
+    message after the idle period starts a fresh session id for that chat key.
+    This does not delete transcripts - it just starts a new session.
 
     ```json5
     {
@@ -2953,23 +2954,18 @@ Related: [/concepts/oauth](/concepts/oauth) (OAuth flows, token storage, multi-a
 
     ```json5
     {
-      agents: {
-        defaults: {
-          tools: {
-            message: {
-              crossContext: {
-                allowAcrossProviders: true,
-                marker: { enabled: true, prefix: "[from {channel}] " },
-              },
-            },
+      tools: {
+        message: {
+          crossContext: {
+            allowAcrossProviders: true,
+            marker: { enabled: true, prefix: "[from {channel}] " },
           },
         },
       },
     }
     ```
 
-    Restart the gateway after editing config. If you only want this for a single
-    agent, set it under `agents.list[].tools.message` instead.
+    Restart the gateway after editing config.
 
   </Accordion>