core: clarify that temp directory already exists for AI agents

The bash tool description now explicitly states that the temp directory has already been created and exists, preventing agents from unnecessarily trying to create it before use.

packages/opencode/src/tool/bash.txt

@@ -4,7 +4,7 @@ Be aware: OS: ${os}, Shell: ${shell}
 
 All commands run in the current working directory by default. Use the `workdir` parameter if you need to run a command in a different directory. AVOID using `cd <directory> && <command>` patterns - use `workdir` instead.
 
-Use `${tmp}` for temporary work outside the workspace. This directory is pre-approved for external directory access.
+Use `${tmp}` for temporary work outside the workspace. This directory has already been created, already exists, and is pre-approved for external directory access.
 
 IMPORTANT: This tool is for terminal operations like git, npm, docker, etc. DO NOT use it for file operations (reading, writing, editing, searching, finding files) - use the specialized tools for this instead.