Config: add MiniMax direct API authentication option

Makes apiKey optional in ModelProviderConfig so MiniMax can use auth
profiles or environment variables (MINIMAX_API_KEY) instead of requiring
explicit config.

Changes:
- src/config/types.ts: apiKey changed from required to optional
- src/config/zod-schema.ts: use z.string().min(1).optional() for validation
- src/commands/configure.ts: add minimax-api auth choice in wizard
- src/commands/onboard-auth.ts: MiniMax provider omits apiKey (uses env/auth)
- patches/@mariozechner__pi-ai@0.42.1.patch: map minimax → MINIMAX_API_KEY

Auth Resolution Order (unchanged):
1. Auth profiles (highest priority)
2. Environment variables (MINIMAX_API_KEY, etc.)
3. Custom provider apiKey from models.json (lowest priority)

src/commands/onboard-auth.ts

@@ -336,7 +336,7 @@ export function applyMinimaxApiProviderConfig(
   const providers = { ...cfg.models?.providers };
   providers.minimax = {
     baseUrl: MINIMAX_API_BASE_URL,
-    apiKey: "", // Resolved via MINIMAX_API_KEY env var or auth profile
+    // apiKey omitted: resolved via MINIMAX_API_KEY env var or auth profile by default.
     api: "anthropic-messages",
     models: [buildMinimaxApiModelDefinition(modelId)],
   };