fix(coding-agent): refresh thinking border from extensions

closes #3888

packages/coding-agent/docs/extensions.md

@@ -40,6 +40,7 @@ See [examples/extensions/](../examples/extensions/) for working implementations.
   - [Resource Events](#resource-events)
   - [Session Events](#session-events)
   - [Agent Events](#agent-events)
+  - [Model Events](#model-events)
   - [Tool Events](#tool-events)
 - [ExtensionContext](#extensioncontext)
 - [ExtensionCommandContext](#extensioncommandcontext)
@@ -323,8 +324,12 @@ user sends another prompt ◄─────────────────
   └─► session_tree
 
 /model or Ctrl+P (model selection/cycling)
+  ├─► thinking_level_select (if model change changes/clamps thinking level)
   └─► model_select
 
+thinking level changes (settings, keybinding, pi.setThinkingLevel())
+  └─► thinking_level_select
+
 exit (Ctrl+C, Ctrl+D, SIGHUP, SIGTERM)
   └─► session_shutdown
 ```
@@ -649,6 +654,21 @@ pi.on("model_select", async (event, ctx) => {
 
 Use this to update UI elements (status bars, footers) or perform model-specific initialization when the active model changes.
 
+#### thinking_level_select
+
+Fired when the thinking level changes. This is notification-only; handler return values are ignored.
+
+```typescript
+pi.on("thinking_level_select", async (event, ctx) => {
+  // event.level - newly selected thinking level
+  // event.previousLevel - previous thinking level
+
+  ctx.ui.setStatus("thinking", `thinking: ${event.level}`);
+});
+```
+
+Use this to update extension UI when `pi.setThinkingLevel()`, model changes, or built-in thinking-level controls change the active thinking level.
+
 ### Tool Events
 
 #### tool_call
@@ -1502,7 +1522,7 @@ if (model) {
 
 ### pi.getThinkingLevel() / pi.setThinkingLevel(level)
 
-Get or set the thinking level. Level is clamped to model capabilities (non-reasoning models always use "off").
+Get or set the thinking level. Level is clamped to model capabilities (non-reasoning models always use "off"). Changes emit `thinking_level_select`.
 
 ```typescript
 const current = pi.getThinkingLevel();  // "off" | "minimal" | "low" | "medium" | "high" | "xhigh"