devpit
Excalidraw is a capability: hand-drawn diagrams, saved as files in the project. Install it from the Capabilities pane and an Excalidraw row appears in the sidebar.
Drawings
The Excalidraw row opens the project's drawings, each with when it was last edited, and a field for the next one: type a name, press Create, and it opens on the canvas.
The list is read from the folder every time you look, so an .excalidraw file copied in from outside is a drawing like any other. A name already taken is refused rather than written over.
A drawing opens in a tab of its own, on the Excalidraw canvas. Rename drawing on the pane bar renames the file and reopens it under the new name. The canvas follows devpit's theme — light, dark, or whichever the system is using when Appearance is set to System.
Saving
There is no save button. A drawing is written a moment after the last change.
Every write carries the time devpit last read the file, so a drawing something else changed is not overwritten. When that happens the change is not saved and the pane says so:
<name>changed on disk after it was opened, so this change was not saved.
Reload takes what is on disk, Keep mine writes yours over it. While that question is open, closing the tab asks first — it is the one state where closing would lose the drawing.
A drawing file is capped at 32 MiB.
Pinning to a card
Pin to card on the pane bar lists this project's cards; pick one and the drawing is pinned to it, which the pane confirms. The menu is not shown when the project has no cards.
A pin points at the file where it already is — nothing is copied. On the card, opening the pin brings the drawing back up in Excalidraw, as long as the capability is still on.
Where drawings live
In devpit's folder for this project, as .excalidraw files:
~/.devpit/projects/<name>-<suffix>/data/excalidraw/
Not in your repository. Uninstalling Excalidraw leaves them there unless you tick the box that deletes them.