Add page revision history in the shell

## Goal

Cortext already snapshots a revision every \~10 minutes while a page is being edited, and keeps up to 50 of them. The data is there; what's missing is any UI in the shell to look at it. The only way to see past versions or roll back today is wp-admin's revision viewer. Bring page history into the shell.

## Scope

* A "Page history" entry on the page menu that opens a side panel.
* Each row shows the timestamp and the author.
* Pick a revision to preview its content next to the current version.
* Restore a revision from the panel.
* The same panel is reachable from a trashed page (read-only) so people can peek at what was there before deciding to restore.

## Out of scope

* Restoring at block granularity. Restore is whole-page.
* Forking a revision into a separate page.
* Restoring a revision of a trashed page in one shot. Restore the page first, then pick a revision.

## Acceptance criteria

* From any live page, you can open the history panel and see past revisions with timestamps and authors.
* From a trashed page, you can open the same panel and preview past revisions read-only.
* Picking a revision shows you what's different from the current version.
* Restoring a revision rolls the page back, and the restore is itself reversible.

No repository files or tests are named. Start by examining the shell's page menu and the existing wp-admin revision viewer, then trace how live and trashed pages expose revision data. Done means a history side panel supports timestamps, authors, previews, read-only trashed-page access, and reversible whole-page restores.

Written by the indexing model from the issue text.