refactor(contracts): compose DesktopApi from per-domain sub-interfaces

Split the flat 134-method DesktopApi interface into 12 domain
sub-interfaces (companies, agents, tasks, runs, approvals,
communication, threads, credentials, skills, budget, automation,
window) under src/shared/contracts/desktop-api/. desktop-api.ts
now intersects them to preserve the exact external flat shape;
window.agentCompany consumers and preload wiring are untouched.

src/shared/contracts/desktop-api/budget.ts

@@ -0,0 +1,13 @@
+import type { z } from "zod";
+import type { BudgetIncidentRecord, BudgetPolicyRecord, BudgetStatusSummary } from "../../types";
+import type { budgetPolicyInputSchema, deleteBudgetPolicySchema, listBudgetIncidentsSchema, resolveBudgetIncidentSchema } from "../schemas-budget";
+import type { DesktopResult } from "./common";
+
+export type BudgetApi = {
+  saveBudgetPolicy: (input: z.infer<typeof budgetPolicyInputSchema>) => Promise<DesktopResult<string>>;
+  deleteBudgetPolicy: (input: z.infer<typeof deleteBudgetPolicySchema>) => Promise<DesktopResult<boolean>>;
+  listBudgetPolicies: (companyId: string) => Promise<DesktopResult<BudgetPolicyRecord[]>>;
+  listBudgetIncidents: (input: z.infer<typeof listBudgetIncidentsSchema>) => Promise<DesktopResult<BudgetIncidentRecord[]>>;
+  resolveBudgetIncident: (input: z.infer<typeof resolveBudgetIncidentSchema>) => Promise<DesktopResult<boolean>>;
+  getBudgetStatus: (companyId: string) => Promise<DesktopResult<BudgetStatusSummary>>;
+};