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).

server/openapi-extra.js

@@ -19,6 +19,7 @@ const aiTools = require("./openapi-extra/ai-tools");
 const misc = require("./openapi-extra/misc");
 const auth = require("./openapi-extra/auth");
 const files = require("./openapi-extra/files");
+const artifacts = require("./openapi-extra/artifacts");
 // Enriched overrides of already-documented endpoints — same operationId, tags,
 // and request/response `$ref` schemas as the base, with added examples and
 // richer descriptions. Listed last so they win the merge.
@@ -56,6 +57,7 @@ module.exports = combine(
   misc,
   auth,
   files,
+  artifacts,
   overrideSessionsAgents,
   overrideCore,
   overridePricingAlerts,