feat(llm): native Google Gemini provider via OpenAI-compatible endpoint

Add a first-class `gemini` provider so operators can drive missions with a
direct GEMINI_API_KEY, instead of only reaching Gemini through OpenRouter.

Gemini is wired exactly like xAI: dispatched through OpenAIAdapter against
Google's OpenAI-compatible surface. The base URL is
https://generativelanguage.googleapis.com/v1beta/openai — the `/openai`
segment is required because OpenAIAdapter posts to `${baseUrl}/chat/completions`;
without it requests hit `/v1beta/chat/completions`, which Google does not serve.

Wires the provider through the type union, adapter factory, config
(interface, defaults, model catalog, key env map, getLLMConfig, fallback
chain, redaction, .env template), and adds regression tests pinning the
`/openai` base-URL segment.

src/llm/index.ts

@@ -1207,6 +1207,8 @@ export class LLMBackbone extends EventEmitter<LLMEvents> {
         return new OpenAIAdapter(config);
       case 'xai':
         return new OpenAIAdapter(config); // xAI (Grok Build / grok-*) is OpenAI-compatible
+      case 'gemini':
+        return new OpenAIAdapter(config); // Gemini via Google's OpenAI-compatible endpoint (baseUrl ends in /v1beta/openai)
       case 'codex':
         return new CodexAdapter(config);
       case 'mock':