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/views/agent-manager/AgentManagerEmptyState.tsx

@@ -57,7 +57,7 @@ export const AgentManagerEmptyState: React.FC<AgentManagerEmptyStateProps> = ({
   const [prompt, setPrompt] = React.useState('');
   const [selectedModels, setSelectedModels] = React.useState<ModelSelectionWithId[]>([]);
   const [selectedAgent, setSelectedAgent] = React.useState<string>('');
-  const [baseBranch, setBaseBranch] = React.useState('HEAD');
+  const [baseBranch, setBaseBranch] = React.useState('');
   const [attachedFiles, setAttachedFiles] = React.useState<AttachedFile[]>([]);
   const [isSubmitting, setIsSubmitting] = React.useState(false);
   const [setupCommands, setSetupCommands] = React.useState<string[]>([]);
@@ -206,6 +206,7 @@ export const AgentManagerEmptyState: React.FC<AgentManagerEmptyStateProps> = ({
     groupName.trim() && 
     prompt.trim() && 
     selectedModels.length >= 1 && 
+    baseBranch &&
     isGitRepository && 
     !isLoadingBranches
   );