Allow line comments on files outside the thread's workspace

[rmartins90]

I'd like to use the existing line-comment flow on text files outside a thread's workspace.

I often use a coordinating thread rooted in a Markdown vault, which is my shared context layer for agents. From that thread, agents work in the relevant repositories. Those repositories live outside the vault, so files I want to discuss are often outside the coordinating thread's workspace.

For example, when an agent links to a source file in another repository, I can open it in T3 Code. I'd like to select a few lines, add feedback, and send it through the coordinating thread's composer. Today the external file opens read-only, without the line-comment controls available for workspace files.

The proposed scope is to enable those same comments on external text files that T3 Code can already read and display. Each comment would retain the absolute file path, selected lines, and my feedback. The file would stay read-only in the viewer.

In the current implementation, external files use ReadOnlySourcePreview, while commenting lives in EditableFileSurface. This seems like a case for separating commenting from editing.

Related discussions: #16333 covers discovering files outside the workspace, and #12086 covers citing document passages. This proposal focuses on the existing line-comment interaction for external files already open in the viewer.

Would you be open to a PR with that scope?