docs: fix credential resolution order (#80)

Remove incorrect claim that $MINIMAX_API_KEY is part of the resolver
chain. The env var is checked by ensureApiKey() in main.ts before
command execution, and persisted to config.json — it is not directly
resolved by resolveCredential().

Author: raylanlin

docs/cli-design.md

@@ -54,12 +54,10 @@ mmx
 Credential resolution order:
 1. `--api-key` flag (highest priority)
 2. `~/.mmx/credentials.json` (OAuth token — only created by `mmx auth login` without `--api-key`)
-3. `fileApiKey` in `~/.mmx/config.json` (persisted API key)
-4. `$MINIMAX_API_KEY` env var
+3. `fileApiKey` in `~/.mmx/config.json` (persisted API key from a previous `mmx auth login --api-key`)
 
-If none are found, the CLI returns `No credentials found`. The CLI can be fully authenticated
-using any single method — `~/.mmx/credentials.json` is **not required** if you use an API key
-instead.
+If none are found, the CLI returns `No credentials found`. The `~/.mmx/credentials.json` file
+is **not required** when using API key auth — it is only created during OAuth login.
 
 ## Configuration