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/pages/SettingsHub.tsx

@@ -67,6 +67,7 @@ import { SnapshotSettings } from "../components/settings/SnapshotSettings";
 import { ReportSettings } from "../components/settings/ReportSettings";
 import { AiToolsSettings } from "../components/settings/AiToolsSettings";
 import { AgentInstructionsSettings } from "../components/settings/AgentInstructionsSettings";
+import { SessionDefaultsSettings } from "../components/settings/SessionDefaultsSettings";
 import { ArtifactsHubSettings } from "../components/settings/ArtifactsHubSettings";
 import { PricingSettings } from "../components/settings/PricingSettings";
 import { WorktreeVisibilitySettings } from "../components/settings/WorktreeVisibilitySettings";
@@ -292,6 +293,7 @@ export function SettingsHub({ section }: { section?: string } = {}) {
       render: () => (
         <div className="space-y-6">
           <AgentInstructionsSettings />
+          <SessionDefaultsSettings />
           <AiToolsSettings settingsHref={toolHref} />
           {/* The Hub's install switch: server mode, an admin's (it hides itself). */}
           <ArtifactsHubSettings />

client/src/pages/__tests__/__snapshots__/screens.snapshot.test.tsx.snap

@@ -7239,7 +7239,7 @@ exports[`screen snapshots > Settings — about 1`] = `
                 <span
                   class="text-status-warning/90"
                 >
-                  UI build v2.8.11 (rebuild client to match)
+                  UI build v2.8.12 (rebuild client to match)
                 </span>
               </p>
             </div>