WIP: Rewrite export-html with tree sidebar, client-side rendering

- Add tree sidebar with search and filter (Default/All/Labels)
- Client-side markdown/syntax highlighting via vendored marked.js + highlight.js
- Base64 encode session data to avoid escaping issues
- Reuse theme.ts color tokens via getResolvedThemeColors()
- Sticky sidebar, responsive mobile layout with overlay
- Click tree node to scroll to message
- Keyboard shortcuts: Esc to reset, Ctrl/Cmd+F to search

packages/coding-agent/src/core/agent-session.ts

@@ -28,7 +28,7 @@ import {
 	shouldCompact,
 } from "./compaction/index.js";
 import type { CustomToolContext, CustomToolSessionEvent, LoadedCustomTool } from "./custom-tools/index.js";
-import { exportSessionToHtml } from "./export-html.js";
+import { exportSessionToHtml } from "./export-html/index.js";
 import type {
 	HookContext,
 	HookRunner,