docs: add switchSession to ExtensionCommandContext in extensions.md

fixes #2735

packages/coding-agent/docs/extensions.md

@@ -944,6 +944,38 @@ Options:
 - `replaceInstructions`: If true, `customInstructions` replaces the default prompt instead of being appended
 - `label`: Label to attach to the branch summary entry (or target entry if not summarizing)
 
+### ctx.switchSession(sessionPath)
+
+Switch to a different session file:
+
+```typescript
+const result = await ctx.switchSession("/path/to/session.jsonl");
+if (result.cancelled) {
+  // An extension cancelled the switch via session_before_switch
+}
+```
+
+To discover available sessions, use the static `SessionManager.list()` or `SessionManager.listAll()` methods:
+
+```typescript
+import { SessionManager } from "@mariozechner/pi-coding-agent";
+
+pi.registerCommand("switch", {
+  description: "Switch to another session",
+  handler: async (args, ctx) => {
+    const sessions = await SessionManager.list(ctx.cwd);
+    if (sessions.length === 0) return;
+    const choice = await ctx.ui.select(
+      "Pick session:",
+      sessions.map(s => s.file),
+    );
+    if (choice) {
+      await ctx.switchSession(choice);
+    }
+  },
+});
+```
+
 ### ctx.reload()
 
 Run the same reload flow as `/reload`.