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/chat/FileAttachment.tsx

@@ -347,13 +347,13 @@ export const MessageFilesDisplay = memo(({ files, onShowPopup, compact = false }
     return filename || path;
   };
 
-  const resolveDisplayName = (file: FilePart): string => {
+  const resolveDisplayName = React.useCallback((file: FilePart): string => {
     const isGitHubLink = getGitHubLinkKind(file) !== null;
     if (isGitHubLink && typeof file.filename === 'string' && file.filename.trim().length > 0) {
       return file.filename.trim();
     }
     return extractFilename(file.filename || file.url);
-  };
+  }, []);
 
   const formatFileSize = (bytes?: number) => {
     if (!bytes || !Number.isFinite(bytes) || bytes <= 0) return '';
@@ -377,7 +377,7 @@ export const MessageFilesDisplay = memo(({ files, onShowPopup, compact = false }
           size: file.size,
         }];
       }),
-    [imageFiles]
+    [imageFiles, resolveDisplayName]
   );
 
   const handleImageClick = React.useCallback((index: number) => {