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).

package.json

@@ -1,6 +1,6 @@
 {
   "name": "agenthub",
-  "version": "2.8.11",
+  "version": "2.8.12",
   "bin": {
     "agenthub": "bin/agenthub.js"
   },