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/config/types.ts

@@ -1201,7 +1201,7 @@ export type ModelDefinitionConfig = {
 
 export type ModelProviderConfig = {
   baseUrl: string;
-  apiKey: string;
+  apiKey?: string;
   api?: ModelApi;
   headers?: Record<string, string>;
   authHeader?: boolean;

src/config/zod-schema.ts

@@ -40,7 +40,7 @@ const ModelDefinitionSchema = z.object({
 
 const ModelProviderSchema = z.object({
   baseUrl: z.string().min(1),
-  apiKey: z.string().min(1),
+  apiKey: z.string().min(1).optional(),
   api: ModelApiSchema.optional(),
   headers: z.record(z.string(), z.string()).optional(),
   authHeader: z.boolean().optional(),