Export truncation utilities for custom tools, add truncated-tool example

- Export truncateHead, truncateTail, truncateLine, formatSize, DEFAULT_MAX_BYTES, DEFAULT_MAX_LINES from package
- Add examples/extensions/truncated-tool.ts showing proper output truncation with custom rendering
- Document output truncation best practices in docs/extensions.md

packages/coding-agent/src/index.ts

@@ -171,9 +171,12 @@ export {
 	type BashToolDetails,
 	bashTool,
 	codingTools,
+	DEFAULT_MAX_BYTES,
+	DEFAULT_MAX_LINES,
 	editTool,
 	type FindToolDetails,
 	findTool,
+	formatSize,
 	type GrepToolDetails,
 	grepTool,
 	type LsToolDetails,
@@ -182,7 +185,11 @@ export {
 	type ReadToolOptions,
 	readTool,
 	type ToolsOptions,
+	type TruncationOptions,
 	type TruncationResult,
+	truncateHead,
+	truncateLine,
+	truncateTail,
 	writeTool,
 } from "./core/tools/index.js";
 // Main entry point