Undo and Redo are builtins, as in acme's tags, so a pane's ctl and exec can step its body

Over 9P there was no way to take back an edit: acme's Undo and Redo were keys only. They are now pane builtins, reached from a tag, a pane ctl and exec, and listed in /commands. They are not added to the default tags, which would change every tag.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

2 files changed, 6 insertions, 1 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 445aa725..95c1871e 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -144,7 +144,8 @@ takes (`Verbose on`, `WindowOpacity 70`, `PanelSlide off`, `DumpDir` bare
 for the default directory, `LocationsConfig ...`), so writing what it reads
 back changes nothing; platform and startup facts are `/status`'s and the
 Config window's, not settings. A pane's `ctl` takes the builtins that act on
-a pane (`Del`, `Save f`, `Collapse`, which folds that pane, `Find pat`)
+a pane (`Del`, `Save f`, `Collapse`, which folds that pane, `Undo` and
+`Redo`, which step its body through its edits, `Find pat`)
 beside acme's `get`, `lock` and `unlock`. The column words are pane words
 too, acting on the column that pane is in: `Delcol`, `DelAbove`, `DelBelow`
 and the focus moves `Left`/`Right`/`Up`/`Down` from it. `Joincol` and
diff --git a/docs/tags.md b/docs/tags.md
index f1e4a420..de540c91 100644
--- a/docs/tags.md
+++ b/docs/tags.md
@@ -43,6 +43,10 @@ Its text and terminal process are kept;
 the command stays available in the visible tag. Every default pane tag includes
 `Collapse`, including file, terminal, image, and PDF panes.
 
+`Undo` and `Redo` are acme's: typed or clicked in a pane's tag, or written
+to its `ctl`, they step the body back and forward through its edits, as the
+`u` and `U` keys do. They are not in the default tags.
+
 `Del` closes a pane and gives its rows to one neighbor; the rest of the
 column keeps its heights. `Del k` (or `DelAbove`) gives them to the nearest
 expanded pane above, `Del j` (or `DelBelow`) to the one below, each falling