fix(coding-agent): handle stale extension contexts

fixes #3606

packages/coding-agent/docs/extensions.md

@@ -992,14 +992,19 @@ Options:
 Fork from a specific entry, creating a new session file:
 
 ```typescript
-const result = await ctx.fork("entry-id-123");
-if (!result.cancelled) {
-  // Now in the forked session
+const result = await ctx.fork("entry-id-123", {
+  withSession: async (ctx) => {
+    // Use only the replacement-session ctx here.
+    ctx.ui.notify("Now in the forked session", "info");
+  },
+});
+if (result.cancelled) {
+  // An extension cancelled the fork
 }
 
 const cloneResult = await ctx.fork("entry-id-456", { position: "at" });
-if (!cloneResult.cancelled) {
-  // New session contains the active path through entry-id-456
+if (cloneResult.cancelled) {
+  // An extension cancelled the clone
 }
 ```
 
@@ -1060,7 +1065,11 @@ pi.registerCommand("switch", {
       sessions.map(s => s.file),
     );
     if (choice) {
-      await ctx.switchSession(choice);
+      await ctx.switchSession(choice, {
+        withSession: async (ctx) => {
+          ctx.ui.notify("Switched session", "info");
+        },
+      });
     }
   },
 });