File preview: reload only when the open file actually changes

[jameswasher]

The file preview reloads an open HTML page after every finished command, file edit or checkpoint (workspace-revision in FilePreviewPanel.tsx), even when the open file hasn't changed. Any page state is lost, so a slide deck or long report you're reading jumps back to the top while the agent keeps working. Follow-up to #20913.

Idea: keep the refresh signal, but compare the open file before reloading:

• On each workspace mutation, check the open file's mtime or content hash, and remount the frame only if it differs.
• A file_change that names other paths could skip the check entirely; a command_execution (paths unknown) still triggers it.

That keeps today's guarantee (a shell command that rewrites the file still refreshes the preview) without reloading pages nobody touched.

[r3xsean]

+1 — this is really annoying in practice, especially when using T3 Code remotely. Trying to read or interact with an HTML preview while the agent is working becomes frustrating because it keeps reloading after agent actions, even when the open file has not changed. Losing your place or page state repeatedly makes the preview difficult to use alongside an active agent.

Strongly support checking whether the open file actually changed before reloading it. If the contents are unchanged, the preview should stay untouched so we can keep using it while the agent works.