chore: clarify default shell for unified_exec (#8997)

The description of the `shell` arg for `exec_command` states the default
is `/bin/bash`, but AFAICT it's the user's default shell.

Default logic
[here](2a06d64bc9/codex-rs/core/src/tools/handlers/unified_exec.rs (L123)).

EDIT: #9004 has an alternative where we inform the model of the default
shell itself.

codex-rs/core/src/tools/spec.rs

@@ -157,7 +157,7 @@ fn create_exec_command_tool() -> ToolSpec {
         (
             "shell".to_string(),
             JsonSchema::String {
-                description: Some("Shell binary to launch. Defaults to /bin/bash.".to_string()),
+                description: Some("Shell binary to launch. Defaults to the user's default shell.".to_string()),
             },
         ),
         (