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/sidebar/SidebarHeader.tsx

@@ -18,6 +18,7 @@ import {
   RiExpandUpDownLine,
   RiStickyNoteLine,
 } from '@remixicon/react';
+import { ArrowsMerge } from '@/components/icons/ArrowsMerge';
 import type { ProjectRef } from '@/lib/openchamberConfig';
 import { useSessionDisplayStore } from '@/stores/useSessionDisplayStore';
 import { ProjectNotesTodoPanel } from '../ProjectNotesTodoPanel';
@@ -31,6 +32,8 @@ type Props = {
   setProjectNotesPanelOpen: (open: boolean) => void;
   activeProjectRefForHeader: ProjectRef | null;
   activeProjectLabelForHeader: string | null;
+  canOpenMultiRun: boolean;
+  openMultiRunLauncher: () => void;
   stableActiveProjectIsRepo: boolean;
   headerActionIconClass: string;
   reserveHeaderActionsSpace: boolean;
@@ -56,6 +59,8 @@ export function SidebarHeader(props: Props): React.ReactNode {
     setProjectNotesPanelOpen,
     activeProjectRefForHeader,
     activeProjectLabelForHeader,
+    canOpenMultiRun,
+    openMultiRunLauncher,
     stableActiveProjectIsRepo,
     headerActionIconClass,
     reserveHeaderActionsSpace,
@@ -113,6 +118,21 @@ export function SidebarHeader(props: Props): React.ReactNode {
             </div>
 
             <div className="flex items-center gap-1.5">
+              <Tooltip>
+                <TooltipTrigger asChild>
+                  <button
+                    type="button"
+                    onClick={openMultiRunLauncher}
+                    className={headerActionButtonClass}
+                    aria-label="New multi-run"
+                    disabled={!canOpenMultiRun}
+                  >
+                    <ArrowsMerge className={headerActionIconClass} />
+                  </button>
+                </TooltipTrigger>
+                <TooltipContent side="bottom" sideOffset={4}><p>New multi-run</p></TooltipContent>
+              </Tooltip>
+
               {useMobileNotesPanel ? (
                 <Tooltip>
                   <TooltipTrigger asChild>

packages/ui/src/components/session/sidebar/SidebarProjectsList.tsx

@@ -54,7 +54,6 @@ type Props = {
   setSessionSwitcherOpen: (open: boolean) => void;
   openNewSessionDraft: (options?: { directoryOverride?: string | null }) => void;
   openNewWorktreeDialog: () => void;
-  openMultiRunLauncher: () => void;
   openProjectEditDialog: (id: string) => void;
   removeProject: (id: string) => void;
   projectHeaderSentinelRefs: React.MutableRefObject<Map<string, HTMLDivElement | null>>;
@@ -185,10 +184,6 @@ export function SidebarProjectsList(props: Props): React.ReactNode {
                       if (props.mobileVariant) props.setSessionSwitcherOpen(false);
                       props.openNewWorktreeDialog();
                     }}
-                    onOpenMultiRunLauncher={() => {
-                      if (projectKey !== props.activeProjectId) props.setActiveProjectIdOnly(projectKey);
-                      props.openMultiRunLauncher();
-                    }}
                     onRenameStart={() => props.openProjectEditDialog(projectKey)}
                     onClose={() => props.removeProject(projectKey)}
                     sentinelRef={(el) => { props.projectHeaderSentinelRefs.current.set(projectKey, el); }}

packages/ui/src/components/session/sidebar/sortableItems.tsx

@@ -18,7 +18,6 @@ import {
   RiNodeTree,
   RiPencilAiLine,
 } from '@remixicon/react';
-import { ArrowsMerge } from '@/components/icons/ArrowsMerge';
 import { cn } from '@/lib/utils';
 import { PROJECT_COLOR_MAP, PROJECT_ICON_MAP, getProjectIconImageUrl } from '@/lib/projectMeta';
 import { useThemeSystem } from '@/contexts/useThemeSystem';
@@ -43,7 +42,6 @@ export interface SortableProjectItemProps {
   onHoverChange: (hovered: boolean) => void;
   onNewSession: () => void;
   onNewWorktreeSession?: () => void;
-  onOpenMultiRunLauncher: () => void;
   onRenameStart: () => void;
   onClose: () => void;
   sentinelRef: (el: HTMLDivElement | null) => void;
@@ -79,7 +77,6 @@ export const SortableProjectItem: React.FC<SortableProjectItemProps> = ({
   onHoverChange,
   onNewSession,
   onNewWorktreeSession,
-  onOpenMultiRunLauncher,
   onRenameStart,
   onClose,
   sentinelRef,
@@ -267,12 +264,6 @@ export const SortableProjectItem: React.FC<SortableProjectItemProps> = ({
                         New Session
                       </DropdownMenuItem>
                     )}
-                    {showCreateButtons && isRepo && !hideDirectoryControls && (
-                      <DropdownMenuItem onClick={onOpenMultiRunLauncher}>
-                        <ArrowsMerge className="mr-1.5 h-4 w-4" />
-                        New Multi-Run
-                      </DropdownMenuItem>
-                    )}
                     <DropdownMenuItem onClick={onRenameStart}>
                       <RiPencilAiLine className="mr-1.5 h-4 w-4" />
                       Rename