Add OpenAI-compatible LLM client service (v0.19.0)

LlmService wraps the local OpenAI-compatible server (default
http://192.168.1.70:1111/v1, no auth) with native fetch only — no new
dependencies. listModels() -> data[].id; chat(messages, opts?) ->
choices[0].message.content (non-2xx throws with status + body snippet);
summarize(text) = chat() with a concise-summary system prompt.

Env: LLM_API_BASE / LLM_MODEL (qwen3.8-27b) / LLM_TIMEOUT_MS (60000,
AbortSignal.timeout). Plain feature module wired into app.module.ts;
docs synced (AGENTS.md, README env list).

src/app.module.ts

@@ -1,10 +1,17 @@
 import { Module } from '@nestjs/common';
 import { BatteryAlertModule } from './battery/battery.module.js';
 import { CommandModule } from './commands/command.module.js';
+import { LlmModule } from './llm/llm.module.js';
 import { MeshModule } from './mesh/mesh.module.js';
 import { WeatherModule } from './weather/weather.module.js';
 
 @Module({
-  imports: [MeshModule, WeatherModule, BatteryAlertModule, CommandModule],
+  imports: [
+    MeshModule,
+    WeatherModule,
+    BatteryAlertModule,
+    CommandModule,
+    LlmModule,
+  ],
 })
 export class AppModule {}