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/hooks/useChatSearchDirectory.ts

@@ -27,8 +27,8 @@ export const useChatSearchDirectory = (): string | undefined => {
     }
   }
 
-  if (newSessionDraft?.open && newSessionDraft.directoryOverride) {
-    return newSessionDraft.directoryOverride;
+  if (newSessionDraft?.open && (newSessionDraft.bootstrapPendingDirectory || newSessionDraft.directoryOverride)) {
+    return (newSessionDraft.bootstrapPendingDirectory || newSessionDraft.directoryOverride) ?? undefined;
   }
 
   if (activeProjectId) {

packages/ui/src/hooks/useEffectiveDirectory.ts

@@ -39,8 +39,8 @@ export const useEffectiveDirectory = (): string | undefined => {
     }
 
     // If a draft session is open, use its directoryOverride
-    if (newSessionDraft?.open && newSessionDraft.directoryOverride) {
-        return newSessionDraft.directoryOverride;
+    if (newSessionDraft?.open && (newSessionDraft.bootstrapPendingDirectory || newSessionDraft.directoryOverride)) {
+        return (newSessionDraft.bootstrapPendingDirectory || newSessionDraft.directoryOverride) ?? undefined;
     }
 
     // Fall back to the global directory