feat(ai): InferenceProvider interface

Task 11 of the slice plan. Single-method provider contract
(name + generate + healthCheck) so future LAN / external API
implementations can drop in without touching AiWorker.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main/ai/InferenceProvider.ts

@@ -0,0 +1,10 @@
+import type { AiResponse } from './schema.js';
+
+export interface GenerateInput { text: string; }
+export interface HealthResult { ok: boolean; model?: string; reason?: string; }
+
+export interface InferenceProvider {
+  readonly name: string;
+  generate(input: GenerateInput): Promise<AiResponse>;
+  healthCheck(): Promise<HealthResult>;
+}