feat: expose global execution concurrency limit in settings

- Add PUT /api/global-concurrency endpoint to update globalMaxConcurrent
  via CentralCore (validated 1-50 range)
- Make InProcessRuntime semaphore react to live concurrency changes via
  CentralCore "concurrency:changed" event — no restart needed
- Add Global Max Concurrent input to Settings modal Scheduling section
  with fetch-on-mount and save-alongside-project-settings behavior
- Add updateGlobalConcurrency API client function

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/dashboard/app/api.ts

@@ -3191,6 +3191,13 @@ export function fetchGlobalConcurrency(): Promise<GlobalConcurrencyState> {
   return api<GlobalConcurrencyState>("/global-concurrency");
 }
 
+export function updateGlobalConcurrency(updates: { globalMaxConcurrent: number }): Promise<GlobalConcurrencyState> {
+  return api<GlobalConcurrencyState>("/global-concurrency", {
+    method: "PUT",
+    body: JSON.stringify(updates),
+  });
+}
+
 /** Fetch tasks for a specific project */
 export function fetchProjectTasks(projectId: string, limit?: number, offset?: number): Promise<Task[]> {
   const params = new URLSearchParams();