fix(plugins): SIGKILL exec children that ignore SIGTERM

- Escalate timed-out or aborted plugin exec children to SIGKILL after a grace period
- Move ai-explain example to assets/plugins and convert it to CommonJS
- Update plugin docs for the new path and the .js/.mjs module formats

assets/plugins/ai-explain/plugin.json

@@ -0,0 +1,5 @@
+{
+  "name": "AI Explain",
+  "version": "1.0.0",
+  "main": "main.js"
+}