coding-agent: example extension to summarize conversation (#684)

* feat: add summarize extension example

Demonstrates custom UI with markdown rendering and external model
integration for conversation summarization.

* chore: update examples/extensions/README.md with summarize.ts

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- Extension example: `summarize.ts` for summarizing conversations using custom UI and an external model
+
 ## [0.45.3] - 2026-01-13
 
 ## [0.45.2] - 2026-01-13