Desktop app file viewer cannot open links to files under additionalDirectories

## Summary

In the desktop app (Code tab), markdown file links in assistant output open a viewer pane. The viewer only opens files under the session's **primary** working directory. Files under a directory added with `additionalDirectories` (or `--add-dir`) render as clickable links but the viewer shows "Couldn't read this file. It may be unreadable, or the session's connection may have dropped."

## Environment

- Claude Code desktop app, Code tab, Linux host (WSL2), session running in a git worktree as the primary working directory
- `permissions.additionalDirectories` set to a second repository on the same machine
- Observed 15 Sep 2026

## Steps to reproduce

1. Start a session with primary directory `/path/repo-a` and `additionalDirectories: ["/path/repo-b"]`.
2. Have the assistant emit `[doc](/path/repo-b/docs/some.md)` (absolute path).
3. Click the link.

Expected: the viewer opens `/path/repo-b/docs/some.md`, since `repo-b` is a configured working directory the session can read.
Actual: "Couldn't read this file".

## Additional observations

- A relative link to a file that exists only in the primary worktree (not in the main checkout) opens fine, so the viewer resolves against the primary working directory, not the repo's main checkout.
- A symlink inside the primary directory pointing into `repo-b` is also refused (consistent with the recent symlink-handling hardening).
- Sending the same file with the file-sharing tool works, so the session itself can read it.

## Request

Let the viewer open files under any configured working directory (`additionalDirectories`), or document the supported link form for them. Multi-repo setups where one repo holds shared tooling and specs, and the other holds the code, need clickable references across the two.