chore: v2.8.12 — clone queue and progress, session defaults, chat agents, drops as @ mentions

Whole tree from parallel sessions, plus the version bump:
- Clone: a queue with live progress (Add workspace and extra folders),
  clones run under nice/ionice so the machine stays responsive.
- Settings → AI tools → Session defaults: a new session's default
  permission mode / bypass per account.
- Chat lists the background subagents that have not reported back yet.
- A file pasted or dropped into a claude pane goes in as an @ mention.
- Changes panel: say why amend/undo is refused on a pushed commit.
- openapi.yaml regenerated (Artifacts, Host, resumeById).

client/src/components/lanes/LaneChangesPanel.tsx

@@ -1728,7 +1728,12 @@ export function LaneChangesPanel({
               {commits?.[0]?.sha === commitCtx.menu.item.sha && (
                 <MenuItem
                   danger
-                  disabled={busy || opOpen || commitCtx.menu.item.parents.length !== 1}
+                  // Pushed: the server refuses it (EPUSHED), so say why here
+                  // instead of failing after the confirm.
+                  disabled={
+                    busy || opOpen || headPushed || commitCtx.menu.item.parents.length !== 1
+                  }
+                  title={headPushed ? t("changes.undoCommitPushed") : undefined}
                   testId="lane-changes-menu-undo-commit"
                   onSelect={pick(
                     commitCtx.close,