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/NewWorktreeDialog.tsx

@@ -868,15 +868,11 @@ Nice-to-have:
       }
       
       toast.success('Worktree created', {
-        description: `${metadata.branch || metadata.name}${sourceLabel ? ` from ${sourceLabel}` : ''}`,
+        description: `${metadata.branch || metadata.name}${sourceLabel ? ` from ${sourceLabel}` : ''} - bootstrapping in background`,
       });
 
-      try {
-        await loadSessions();
-      } catch {
-        // best effort
-      }
-      
+      void loadSessions().catch(() => undefined);
+
       onOpenChange(false);
 
       if (createdSessionId) {