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)

patches/@mariozechner__pi-ai@0.42.1.patch

@@ -55,3 +55,16 @@ index 20fb0a22aaa28f7ff7c2f44a8b628fa1d9d7d936..0bf46bfb4a6fac5a0304652e42566b2c
                          const reasoningItem = JSON.parse(block.thinkingSignature);
                          output.push(reasoningItem);
                      }
+
+diff --git a/dist/stream.js b/dist/stream.js
+index 0000000..2222222 100644
+--- a/dist/stream.js
++++ b/dist/stream.js
+@@ -43,6 +43,7 @@ export function getEnvApiKey(provider) {
+         xai: "XAI_API_KEY",
+         openrouter: "OPENROUTER_API_KEY",
+         zai: "ZAI_API_KEY",
++        minimax: "MINIMAX_API_KEY",
+         mistral: "MISTRAL_API_KEY",
+     };
+     const envVar = envMap[provider];