[Feature]: Make the default action for file and pull request links configurable

Before submitting

• I searched existing issues and did not find a duplicate.

Area

apps/web

Problem or use case

Clicking a file link or recognized pull request link opens it in T3 Code's right panel. Opening the same target in the preferred editor or system browser requires holding Cmd/Ctrl while clicking.

I use my external editor and GitHub as the primary destinations, so holding a modifier for every link adds friction. The existing "Open links in" setting covers ordinary chat and terminal links, but does not change the primary action for file links or recognized pull request links.

Proposed solution

Make the primary click destination configurable while preserving today's behavior as the default:

• File links: T3 Code or Preferred editor
• Pull request links: T3 Code or Default browser

When an external destination is selected, plain click should open externally and Cmd/Ctrl-click should perform the alternate in-app action.

Apply the preference consistently to chat links, diff filenames, file-browser entries, and pull request links.

Start in apps/web by tracing the existing link behavior and the "Open links in" setting across chat links, diff filenames, file-browser entries, and recognized pull request links. Done means the primary destinations are configurable, today’s in-app behavior remains the default, and Cmd/Ctrl-click performs the alternate action consistently.