File window silently fails to open a linked file outside the working directory

Title: File window silently fails to open a linked file outside the working directory

## Description
When the running task references a skill file (e.g. a step that says
"updating SKILL.md") and shows a clickable link to it, clicking the link
opens nothing if the file lives outside the current working directory
(for example a global skill under `~/.claude/skills//SKILL.md`).
There is no error message and no explanation. "Show in explorer" on the
same path finds the file without issue, confirming the file exists and
the failure is specific to the in-app file window.

## Steps to reproduce
1. Start a task where Claude Code reads a global skill file, e.g.
`C:\Users\\.claude\skills\\SKILL.md`.
2. Expand the collapsible step list under the task summary (e.g. "Ran 6
commands, read SKILL.md, ran an agent, used a tool").
3. Click the "Read SKILL.md" step to open the file it links to.
4. The file window does not open the file. No error is shown.
5. Right-click the same path and choose "Show in explorer" — the file
opens fine in File Explorer.

See attached screenshot: the "Read SKILL.md" step (highlighted) is the
one that fails to open.

## Expected behavior
Either:
- Open the file anyway (the app already has filesystem access to it), or
- Show a clear message, e.g. "This file is outside the current working
directory. Open it anyway?" with a one-click option to proceed.

## Actual behavior
The click is silently a no-op. Nothing indicates why, so it looks like
the app is broken rather than enforcing an intentional restriction.

## Environment
- Claude Code desktop app, Windows 11
- Example path that fails: `C:\Users\Heather Gray\.claude\skills\teach\SKILL.md`
- Working directory at the time: a project worktree unrelated to the
skill's location

Reproduce on Windows 11 by clicking the “Read SKILL.md” step for a linked file outside the working directory, then compare it with “Show in explorer” for the same path. Done means the file window opens the global SKILL.md or shows a clear explanation and one-click option instead of silently doing nothing.

Written by the indexing model from the issue text.