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/worktrees/worktreeManager.ts

@@ -5,6 +5,10 @@ import {
   deleteRemoteBranch,
   git,
 } from '@/lib/gitApi';
+import {
+  clearWorktreeBootstrapState,
+  markWorktreeBootstrapPending,
+} from '@/lib/worktrees/worktreeBootstrap';
 import type {
   CreateGitWorktreePayload,
   GitWorktreeValidationResult,
@@ -241,6 +245,8 @@ export async function createWorktree(project: ProjectRef, args: CreateWorktreeAr
     label: returnedBranch || returnedName,
   };
 
+  markWorktreeBootstrapPending(metadata.path);
+
   return metadata;
 }
 
@@ -268,6 +274,8 @@ export async function removeProjectWorktree(project: ProjectRef, worktree: Workt
     throw new Error('Worktree removal failed');
   }
 
+  clearWorktreeBootstrapState(worktree.path);
+
   const branchName = (worktree.branch || '').replace(/^refs\/heads\//, '').trim();
   if (deleteRemote && branchName) {
     await deleteRemoteBranch(projectDirectory, { branch: branchName, remote: remoteName }).catch(() => undefined);