fix(coding-agent): chain tool_result extension patches

fixes #1280

packages/coding-agent/docs/extensions.md

@@ -524,6 +524,11 @@ pi.on("tool_call", (event) => {
 
 Fired after tool executes. **Can modify result.**
 
+`tool_result` handlers chain like middleware:
+- Handlers run in extension load order
+- Each handler sees the latest result after previous handler changes
+- Handlers can return partial patches (`content`, `details`, or `isError`); omitted fields keep their current values
+
 ```typescript
 import { isBashToolResult } from "@mariozechner/pi-coding-agent";