docs(text): drop MiniMax-M2.7-highspeed from example commands

MiniMax-M2.7-highspeed is no longer the recommended fast variant in the M3 era.
Reword the chat/repl examples to demonstrate --system without --model, letting
the new default (M3) apply. Help descriptions already reference M3 only.

Follow-up to the previous commit on this branch; no code or test changes.

Author: raylanlin

src/commands/text/chat.ts

@@ -171,7 +171,7 @@ export default defineCommand({
   ],
   examples: [
     'mmx text chat --message "What is MiniMax?"',
-    'mmx text chat --model MiniMax-M2.7-highspeed --system "You are a coding assistant." --message "Write fizzbuzz in Python"',
+    'mmx text chat --system "You are a coding assistant." --message "Write fizzbuzz in Python"',
     'mmx text chat --message "Hello" --message "assistant:Hi!" --message "How are you?"',
     'cat conversation.json | mmx text chat --messages-file - --stream',
     'mmx text chat --message "Hello" --output json',

src/commands/text/repl.ts

@@ -302,7 +302,7 @@ export default defineCommand({
   ],
   examples: [
     'mmx text repl',
-    'mmx text repl --model MiniMax-M2.7-highspeed --system "You are a coding assistant."',
+    'mmx text repl --system "You are a coding assistant."',
     'mmx text repl --temperature 0.7 --max-tokens 8192',
   ],
   async run(config: Config, flags: GlobalFlags) {