feat(minimax): Add full MiniMax M2.1 support

- Add MiniMax settings template (config/base-minimax.settings.json)
- Add MiniMax pricing (3 models: M2.1, M2.1-lightning, M2)
- Add validateMiniMaxKey() pre-flight validator
- Integrate MiniMax validation in main CLI flow
- Fix model name casing: M2.1-lightning (lowercase l)

Implementation follows GLM pattern:
- Anthropic-compatible API (no proxy needed)
- Pre-flight validation with health check
- Fail-open on network errors
- Actionable error messages

config/base-minimax.settings.json

@@ -0,0 +1,10 @@
+{
+  "env": {
+    "ANTHROPIC_BASE_URL": "https://api.minimax.io/anthropic",
+    "ANTHROPIC_AUTH_TOKEN": "YOUR_MINIMAX_API_KEY_HERE",
+    "ANTHROPIC_MODEL": "MiniMax-M2.1",
+    "ANTHROPIC_DEFAULT_OPUS_MODEL": "MiniMax-M2.1",
+    "ANTHROPIC_DEFAULT_SONNET_MODEL": "MiniMax-M2.1",
+    "ANTHROPIC_DEFAULT_HAIKU_MODEL": "MiniMax-M2.1-lightning"
+  }
+}