[BUG] Share link export only includes active file instead of full folder context

# Exported share link only includes active file instead of full annotated folder file tree

## Summary

When exporting a Plannotator session to a share link, only the file currently highlighted/open in the editor appears in the exported share link, even though the local `/plannotator-annotate path/to/folder` command correctly loads and displays the full folder file tree.

## What I'm doing

I'm running Plannotator locally against a folder:

```bash
/plannotator-annotate path/to/folder
```

The local Plannotator UI correctly shows the full file tree for that folder.

However, when I export the session to a shareable link, the exported view only contains or displays the file that was highlighted/open in the editor at the time of export.

## Expected behaviour

The exported share link should include the same file tree/context that is visible locally when running Plannotator against a folder.

Ideally, the share link should preserve:

* The full folder file tree
* All files included in the annotated folder context
* The currently selected/open file
* Any annotations or plan context related to the folder

## Actual behaviour

The exported share link only shows the currently highlighted/open file from the editor.

The rest of the folder/file tree context appears to be missing from the shared export.

## Use case

My specific use case is sharing an OpenSpec change with my team for review.

An OpenSpec change is usually spread across multiple files, for example the proposal, design notes, task list, and one or more spec files. When I run Plannotator locally against the OpenSpec change folder, I can see the full file tree and navigate the complete change context.

However, when I export that session to a share link, reviewers only see the currently highlighted/open file. This means they lose the surrounding OpenSpec context and cannot properly review the change as a whole.

For team review, the share link needs to preserve the full folder/file tree so people can inspect the complete OpenSpec change, not just one file.

## Why this matters

When using Plannotator to review or annotate a multi-file change, the exported link needs to preserve the surrounding file context. Without the full tree, the share link loses a lot of value because reviewers can only see one file instead of the complete folder/project context.