feat: Add MiniMax Anthropic-compatible API support (minimax-api)

Add --auth-choice minimax-api for direct MiniMax API usage at
https://api.minimax.io/anthropic using the anthropic-messages API.

Changes:
- Add applyMinimaxApiConfig() function with provider/model config
- Add minimax-api to AuthChoice type and CLI options
- Add handler and non-interactive support
- Fix duplicate minimax entry in envMap
- Update live test to use anthropic-messages API
- Add 11 unit tests covering all edge cases
- Document configuration in gateway docs

Test results:
- 11/11 unit tests pass
- 1/1 live API test passes (verified with real API key)

Co-Authored-By: Claude <noreply@anthropic.com>

src/commands/onboard-types.ts

@@ -16,6 +16,7 @@ export type AuthChoice =
   | "gemini-api-key"
   | "minimax-cloud"
   | "minimax"
+  | "minimax-api"
   | "skip";
 export type GatewayAuthChoice = "off" | "token" | "password";
 export type ResetScope = "config" | "config+creds+sessions" | "full";