docs: clarify missing environment sandbox creation

The exec explanation introduced interactive stdin before explaining the missing sandbox case. Introduce that case first, then explain when creation is offered and how approval and lifecycle commands apply.

content/manuals/ai/sandboxes/configuration/environment-files.md

@@ -153,11 +153,11 @@ Both commands follow the [plan approval process](#review-an-environment-plan)
 and run `initialize` [lifecycle commands](#lifecycle). After creating a sandbox,
 they run `postCreate` commands before returning or attaching.
 
-To run a command in the sandbox, use `sbx env exec`. With interactive stdin,
-`exec` offers to create a missing sandbox through the same approval and lifecycle
-process before running your command. For an existing sandbox, it skips lifecycle
-commands. With noninteractive stdin or `--no-input`, create the sandbox first
-with `sbx env create`. See
+To run a command in the sandbox, use `sbx env exec`. If the sandbox is missing,
+`exec` offers to create it when stdin is interactive. Creation follows the same
+approval and lifecycle process before running your command. For an existing
+sandbox, `exec` skips lifecycle commands. With noninteractive stdin or
+`--no-input`, create the sandbox first with `sbx env create`. See
 [Reuse an environment in automation](#reuse-an-environment-in-automation).
 
 ### Set user defaults