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/src/bridge.ts

@@ -3323,6 +3323,24 @@ export async function handleBridgeMessage(message: BridgeRequest, ctx?: BridgeCo
         return { id, type, success: true, data: result };
       }
 
+      case 'api:git/worktrees/bootstrap-status': {
+        const { directory } = (payload || {}) as { directory?: string };
+        if (!directory) {
+          return { id, type, success: false, error: 'Directory is required' };
+        }
+        const result = await gitService.getWorktreeBootstrapStatus(directory);
+        return { id, type, success: true, data: result };
+      }
+
+      case 'api:git/worktrees/preview': {
+        const { directory } = (payload || {}) as { directory?: string };
+        if (!directory) {
+          return { id, type, success: false, error: 'Directory is required' };
+        }
+        const result = await gitService.previewWorktreeCreate(directory, (payload || {}) as gitService.CreateGitWorktreePayload);
+        return { id, type, success: true, data: result };
+      }
+
       case 'api:git/diff': {
         const { directory, path: filePath, staged, contextLines } = (payload || {}) as { 
           directory?: string;