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/vscode/webview/api/git.ts

@@ -142,6 +142,20 @@ export const createVSCodeGitAPI = (): GitAPI => ({
     });
   },
 
+  getGitWorktreeBootstrapStatus: async (directory: string): Promise<{ status: 'pending' | 'ready' | 'failed'; error: string | null; updatedAt: number }> => {
+    return sendBridgeMessage<{ status: 'pending' | 'ready' | 'failed'; error: string | null; updatedAt: number }>('api:git/worktrees/bootstrap-status', {
+      directory,
+    });
+  },
+
+  previewGitWorktree: async (directory: string, payload: CreateGitWorktreePayload): Promise<GitWorktreeCreateResult> => {
+    return sendBridgeMessage<GitWorktreeCreateResult>('api:git/worktrees/preview', {
+      directory,
+      method: 'POST',
+      ...(payload || {}),
+    });
+  },
+
   createGitWorktree: async (directory: string, payload: CreateGitWorktreePayload): Promise<GitWorktreeCreateResult> => {
     return sendBridgeMessage<GitWorktreeCreateResult>('api:git/worktrees', {
       directory,