The directory browser could only descend from a manually-typed path
since the SDK had no way to enumerate the server's filesystem roots
(multiple drives on Windows, mount points, home dir). Add
file.roots() to sdk.ts (GET /file/roots, added server-side in
dzianisv/opencode#238) and show the results as pinned top-level chips
in DirectoryBrowserSheet that jump straight into that root.
Degrades gracefully: older servers 404 on the new endpoint, which the
SDK turns into null, normalizeRoots() turns into an empty list, and
the browser just shows no chips — manual "Jump to path" entry keeps
working exactly as before.
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
