feat(HAI-002): add maxWorktrees setting to limit total git worktrees

- Add maxWorktrees field to Settings interface with default of 4
- Enforce worktree limit in Scheduler.schedule() alongside maxConcurrent
- Wire setting through dashboard startup from persisted settings
- Include maxWorktrees in /api/config response
- Add Max Worktrees input to Settings Modal UI
- Backward compatible: existing configs without maxWorktrees use default

packages/core/src/types.ts

@@ -43,11 +43,13 @@ export interface TaskCreateInput {
 
 export interface Settings {
   maxConcurrent: number;
+  maxWorktrees: number;
   pollIntervalMs: number;
 }
 
 export const DEFAULT_SETTINGS: Settings = {
   maxConcurrent: 2,
+  maxWorktrees: 4,
   pollIntervalMs: 15000,
 };