feat: add clickable markdown file references

[stenci]

Issue for this PR

Closes #13430

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Makes file references in markdown messages clickable in the session view.

When a reference matches a single file, it opens that file directly. When it matches multiple files, it shows a picker. References with line numbers or ranges also highlight the target lines, and external file paths are supported too.

I’ve been using this locally for the past few weeks and found it to be a real time-saver when jumping from chat output to the referenced changes with a single click.

How did you verify your code works?

Tested locally by clicking file references in assistant messages and confirming that:

• unique paths open immediately
• ambiguous filename-only references show a file picker
• file:line highlights the target line
• file:start-end highlights the full range
• external file paths open correctly

Screenshots / recordings

Attached a video showing the behaviors above.
https://github.com/user-attachments/assets/05be6ba1-04ae-4790-ad50-268928a5fbfd

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found:

• PR feat(ui): make edited file references clickable in desktop session UI #15269: "feat(ui): make edited file references clickable in desktop session UI"
  • feat(ui): make edited file references clickable in desktop session UI #15269

Why it might be related: PR #15269 appears to address a very similar feature - making file references clickable in the session UI. Both PRs are focused on enabling clickable file references in markdown messages. This could be an earlier attempt at the same feature or an incomplete implementation that the current PR (18836) is improving upon.

[stenci]

This definitely overlaps with #15269, but this PR is intentionally narrower and aimed specifically at #13430.

It focuses on clickable file refs that show up in markdown in the session UI, including assistant messages and markdown-rendered tool outputs. It also handles direct opens, ambiguous matches with a picker, line/range highlighting, and external paths.

From what I can tell, #15269 also goes further into custom tool-card file links and broader desktop/open-path plumbing. I kept this one smaller and more focused so it’s closer to the web UI issue and easier to review.