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/sidebar/SidebarProjectsList.tsx

@@ -54,7 +54,6 @@ type Props = {
   setSessionSwitcherOpen: (open: boolean) => void;
   openNewSessionDraft: (options?: { directoryOverride?: string | null }) => void;
   openNewWorktreeDialog: () => void;
-  openMultiRunLauncher: () => void;
   openProjectEditDialog: (id: string) => void;
   removeProject: (id: string) => void;
   projectHeaderSentinelRefs: React.MutableRefObject<Map<string, HTMLDivElement | null>>;
@@ -185,10 +184,6 @@ export function SidebarProjectsList(props: Props): React.ReactNode {
                       if (props.mobileVariant) props.setSessionSwitcherOpen(false);
                       props.openNewWorktreeDialog();
                     }}
-                    onOpenMultiRunLauncher={() => {
-                      if (projectKey !== props.activeProjectId) props.setActiveProjectIdOnly(projectKey);
-                      props.openMultiRunLauncher();
-                    }}
                     onRenameStart={() => props.openProjectEditDialog(projectKey)}
                     onClose={() => props.removeProject(projectKey)}
                     sentinelRef={(el) => { props.projectHeaderSentinelRefs.current.set(projectKey, el); }}