Subagent: markdown rendering in expanded view, chain streaming shows all steps

- Export getMarkdownTheme() from coding-agent for custom tools
- Chain/parallel modes now use Markdown component in expanded view
- Chain streaming updates include all previous steps (not just current)
- Strip {previous} placeholder from task preview in renderCall

packages/coding-agent/src/index.ts

@@ -122,6 +122,7 @@ export {
 	type TruncationResult,
 	writeTool,
 } from "./core/tools/index.js";
-
 // Main entry point
 export { main } from "./main.js";
+// Theme utilities for custom tools
+export { getMarkdownTheme } from "./modes/interactive/theme/theme.js";