feat: instant draft-first worktree creation and multi-run launcher redesign (#741)

## Summary

- **Instant worktree creation from chat draft**: selecting "+ New worktree" in the draft branch selector immediately creates a session draft and bootstraps the worktree in the background — no modal interruption
- **Redesigned multi-run launcher**: compact 2-column grid layout in a right-sized dialog with scroll shadow, sticky footer, tooltips replacing verbose descriptions, and project icons in the selector
- **Branch selector aligned across surfaces**: multi-run and agent manager branch pickers now use the shared git store and match NewWorktreeDialog behavior (same default resolution cascade, no synthetic HEAD option, all branches shown)
- **Opaque model multi-select dropdown**: fixes text bleed-through on translucent backgrounds by compositing `--surface-elevated` over `--surface-background`
- **"+ New" inline button in sidebar worktree headers** for faster worktree creation

## Why

Worktree creation was behind modal flow that interrupted the user's train of thought. The draft-first approach lets users start typing immediately while the worktree bootstraps. The multi-run launcher had an oversized form layout with redundant explanations, and its branch picker behaved differently from the main worktree dialog - causing confusion about which branches were available and what the default was.

packages/ui/src/components/session/SessionSidebar.tsx

@@ -1293,6 +1293,14 @@ export const SessionSidebar: React.FC<SessionSidebarProps> = ({
     openNewSessionDraft();
   }, [mobileVariant, openNewSessionDraft, setActiveMainTab, setSessionSwitcherOpen]);
 
+  const handleOpenMultiRunFromHeader = React.useCallback(() => {
+    setActiveMainTab('chat');
+    if (mobileVariant) {
+      setSessionSwitcherOpen(false);
+    }
+    openMultiRunLauncher();
+  }, [mobileVariant, openMultiRunLauncher, setActiveMainTab, setSessionSwitcherOpen]);
+
   return (
     <div
       ref={sessionSearchContainerRef}
@@ -1336,6 +1344,8 @@ export const SessionSidebar: React.FC<SessionSidebarProps> = ({
         setProjectNotesPanelOpen={setProjectNotesPanelOpen}
         activeProjectRefForHeader={activeProjectRefForHeader}
         activeProjectLabelForHeader={activeProjectLabelForHeader}
+        canOpenMultiRun={projects.length > 0}
+        openMultiRunLauncher={handleOpenMultiRunFromHeader}
         stableActiveProjectIsRepo={stableActiveProjectIsRepo}
         headerActionIconClass={headerActionIconClass}
         reserveHeaderActionsSpace={reserveHeaderActionsSpace}
@@ -1376,7 +1386,6 @@ export const SessionSidebar: React.FC<SessionSidebarProps> = ({
         setSessionSwitcherOpen={setSessionSwitcherOpen}
         openNewSessionDraft={openNewSessionDraft}
         openNewWorktreeDialog={openNewWorktreeDialog}
-        openMultiRunLauncher={openMultiRunLauncher}
         openProjectEditDialog={setEditingProjectDialogId}
         removeProject={removeProject}
         projectHeaderSentinelRefs={projectHeaderSentinelRefs}