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/lib/worktree.js

@@ -19,6 +19,26 @@ const PROTECTED_BRANCHES = new Set(["main", "master"]);
 /** Most status lines `statusCounts` reports by name; the counts stay exact. */
 const STATUS_FILE_CAP = 20;
 
+/**
+ * `nice`/`ionice` before a heavy git (a clone) so the dashboard and the panes
+ * beside it keep their share of the machine; children (index-pack, sudo's
+ * git) inherit both. Empty where neither exists (Windows).
+ */
+let lowPriorityPrefix = null;
+function lowPriority() {
+  if (lowPriorityPrefix) return lowPriorityPrefix;
+  const find = (name) =>
+    ["/usr/bin", "/bin"].map((d) => path.join(d, name)).find((f) => fs.existsSync(f));
+  const nice = process.platform === "win32" ? null : find("nice");
+  const ionice = process.platform === "linux" ? find("ionice") : null;
+  lowPriorityPrefix = [
+    ...(nice ? [nice, "-n", "10"] : []),
+    // best-effort class, lowest level: never the idle class, which can wait forever
+    ...(ionice ? [ionice, "-c", "2", "-n", "7"] : []),
+  ];
+  return lowPriorityPrefix;
+}
+
 /**
  * Promisified git wrapper. On failure, throws an Error with err.git = { args, code, stderr }.
  * Treats zero exit code as success even if stderr has hints.
@@ -42,6 +62,10 @@ async function git(
     encoding = "utf8",
     // stdin (a patch for `apply -`): never a file the dashboard writes.
     input = null,
+    // true: run under nice/ionice (a clone — see lowPriority above).
+    background = false,
+    // Called with each stderr chunk as it arrives (a clone's --progress).
+    onStderr = null,
   } = {}
 ) {
   // Build a clean environment: copy process.env but scrub git hook variables
@@ -88,6 +112,12 @@ async function git(
     if (literal) pass.GIT_LITERAL_PATHSPECS = "1";
     [cmd, argv] = require("./run-as").sudoArgv(member, "git", args, pass);
   }
+  const gitCmd = cmd;
+  if (background && lowPriority().length) {
+    const [first, ...rest] = lowPriority();
+    argv = [...rest, cmd, ...argv];
+    cmd = first;
+  }
 
   try {
     const run =
@@ -100,7 +130,7 @@ async function git(
               );
               child.stdin.end(input);
             });
-    const result = await run(cmd, argv, {
+    const pending = run(cmd, argv, {
       cwd,
       env,
       // A read with its own size cap passes a bigger buffer and treats an
@@ -113,12 +143,23 @@ async function git(
       timeout,
       encoding,
     });
+    // promisify(execFile) hands back its child; the stdin runner has none.
+    if (onStderr && pending.child?.stderr) {
+      pending.child.stderr.on("data", (d) => {
+        try {
+          onStderr(String(d));
+        } catch {
+          /* a progress reader never fails the git call */
+        }
+      });
+    }
+    const result = await pending;
     return { stdout: result.stdout, stderr: result.stderr };
   } catch (err) {
     // ENOENT from execFile is ambiguous: a missing binary OR a missing cwd.
     // With the folder present it is git itself that is absent — say so, with
     // the fix, rather than `git status failed` (like ETMUXMISSING for tmux).
-    if (err.code === "ENOENT" && cmd === "git" && fs.existsSync(cwd)) {
+    if (err.code === "ENOENT" && gitCmd === "git" && fs.existsSync(cwd)) {
       const error = new Error(
         "git is not installed (or not on the dashboard's PATH), and every git action needs it. " +
           "Install it (sudo apt install git / brew install git / https://git-scm.com/download/win), " +