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/multirun/AgentSelector.tsx

@@ -7,6 +7,7 @@ import {
   SelectTrigger,
   SelectValue,
 } from '@/components/ui/select';
+import { cn } from '@/lib/utils';
 import { useConfigStore } from '@/stores/useConfigStore';
 
 export interface AgentSelectorProps {
@@ -85,7 +86,14 @@ export const AgentSelector: React.FC<AgentSelectorProps> = ({
       <SelectTrigger
         id={id}
         size="lg"
-        className={className ?? 'max-w-full typography-meta text-foreground'}
+        className={cn(
+          'max-w-full typography-meta text-foreground !border-border/80 !bg-[var(--surface-subtle)]/95 !backdrop-blur-sm hover:!bg-[var(--interactive-hover)]/70 data-[state=open]:!bg-[var(--interactive-active)]/70',
+          className,
+        )}
+        style={{
+          backdropFilter: 'blur(10px)',
+          WebkitBackdropFilter: 'blur(10px)',
+        }}
       >
         <SelectValue placeholder="Select an agent" />
       </SelectTrigger>