desktop: file links in messages 404 when referenced by basename outside the session root #50876

Summary

Clicking a file link in a session message shows a load failure for files that are not
at the session directory root. References written as bare filenames (for example
cleaned.csv) are resolved only against the session directory, so files that live in a
subdirectory of the project always return 404, and the UI shows only a generic error.

Environment

• opencode version: 2.0.12 (desktop app, .deb install at /opt/OpenCode)
• OS: Linux 6.17.0-14-generic, Ubuntu 24.04.4 LTS, x86_64, Wayland (GNOME)
• Terminal: N/A (Electron desktop app)
• Shell: /usr/bin/zsh
• Install/channel: latest; update to 2.0.15 downloaded and staged
• Active plugins: none found in config

Reproduction

1. In a project session (session directory = project root), have the agent write outputs
   into a subdirectory, for example procrastination_analysis/REPORT.md,
   procrastination_analysis/cleaned.csv, and procrastination_analysis/figs/.
2. The assistant answer references them by bare name: cleaned.csv, REPORT.md, figs.
3. Click one of those links in the desktop app.

Expected Behavior

The file opens. The app should resolve the file's real location (or at least show the
resolved path in the error). A directory reference should open a listing.

Actual Behavior

The request returns 404 and the UI only reports a generic load failure. Server log:

GET /api/fs/read/cleaned.csv                          404
GET /api/fs/read/REPORT.md                            404
GET /api/fs/read/figs                                 404
GET /api/fs/read/procrastination_survey_dataset.csv   404

Additional Context

/api/fs/read only resolves a path relative to the requested location, so the same file
succeeds when the correct relative path is used. Running the server directly:

GET /api/fs/read/cleaned.csv?location[directory]=<project root>                          -> 404
GET /api/fs/read/procrastination_analysis/cleaned.csv?location[directory]=<project root> -> 200
GET /api/fs/read/procrastination_survey_dataset.csv?location[directory]=$HOME            -> 404
GET /api/fs/read/下载/procrastination_survey_dataset.csv  (default location = $HOME)     -> 200

Notes:

• /api/fs/read cannot serve a directory at all (figs returns 404 even though it exists).
• A file outside the project (for example in ~/下载) referenced by bare filename also
  404s, because the default location is $HOME.
• Reproducible consistently, not intermittent.
• The response body for these 404s is a FileNotFoundError with the unresolved path,
  but the desktop UI does not surface it.

Suggested improvements:

• Fall back to resolving a bare filename under the session root (and under its subdirectories).
• Show the resolved path in the error instead of a generic load failure.
• Let directory references open the file tree / listing instead of 404ing.