fix(docs): remove stale --format=prompt from README CLI examples [hx-19bf2627]

The --format=prompt value was removed in 0e8e8b4 (hx-4e05d540).
README.md still listed it as a planned option, causing confusion.
Remaining documented values (llm, json) match the actual CLI.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: easel

README.md

@@ -52,7 +52,6 @@ dun check
 Planned options:
 
 ```bash
-dun check --format=prompt
 dun check --format=llm
 dun check --format=json
 dun check --automation=plan