Restructure extensions.md: add Custom Tools section, improve sendMessage docs

- Add ## Custom Tools section with Tool Definition, Multiple Tools, Custom Rendering
- Expand pi.sendMessage docs with deliverAs mode explanations
- Add Extension Styles subsection (single file, dir, package.json)
- Expand ExtensionCommandContext with waitForIdle, newSession, branch, navigateTree
- Replace lodash with zod in examples
- Various fixes and reorganization

packages/coding-agent/CHANGELOG.md

@@ -37,7 +37,7 @@ Hooks and custom tools are now unified as **extensions**. Both were TypeScript m
 // extensions/my-package/package.json
 {
   "name": "my-extension-package",
-  "dependencies": { "lodash": "^4.0.0" },
+  "dependencies": { "zod": "^3.0.0" },
   "pi": {
     "extensions": ["./src/main.ts", "./src/tools.ts"]
   }