revert #386 due to unsafe shell command parsing (#478)

Reverts https://github.com/openai/codex/pull/386 because:

* The parsing logic for shell commands was unsafe (`split(/\s+/)`
instead of something like `shell-quote`)
* We have a different plan for supporting auto-approved commands.

README.md

@@ -289,9 +289,6 @@ model: o4-mini # Default model
 approvalMode: suggest # or auto-edit, full-auto
 fullAutoErrorMode: ask-user # or ignore-and-continue
 notify: true # Enable desktop notifications for responses
-safeCommands:
-  - npm test # Automatically approve npm test
-  - yarn lint # Automatically approve yarn lint
 ```
 
 ```json