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/lib/gitApi.ts

@@ -453,6 +453,33 @@ export async function validateGitWorktree(
   return gitHttp.validateGitWorktree(directory, payload);
 }
 
+export async function getGitWorktreeBootstrapStatus(
+  directory: string,
+): Promise<import('./api/types').GitWorktreeBootstrapStatus> {
+  const runtime = getRuntimeGit();
+  if (runtime?.worktree?.bootstrapStatus) {
+    return runtime.worktree.bootstrapStatus(directory);
+  }
+  if (runtime?.getGitWorktreeBootstrapStatus) {
+    return runtime.getGitWorktreeBootstrapStatus(directory);
+  }
+  return gitHttp.getGitWorktreeBootstrapStatus(directory);
+}
+
+export async function previewGitWorktree(
+  directory: string,
+  payload: import('./api/types').CreateGitWorktreePayload
+): Promise<import('./api/types').GitWorktreeCreateResult> {
+  const runtime = getRuntimeGit();
+  if (runtime?.worktree?.preview) {
+    return runtime.worktree.preview(directory, payload);
+  }
+  if (runtime?.previewGitWorktree) {
+    return runtime.previewGitWorktree(directory, payload);
+  }
+  return gitHttp.previewGitWorktree(directory, payload);
+}
+
 export async function createGitWorktree(
   directory: string,
   payload: import('./api/types').CreateGitWorktreePayload