Added MacOS specific instructions.

CLAUDE.md

@@ -36,3 +36,25 @@ I'm an engineer. I know what I want. You must not propose next actions, unless I
 
 - Do not use any emojis nor fancy characters (non-ASCII outside of range 0x00 - 0x7f) when creating or modifying content. Do not scan files each time you touch them, just use the right characters.
 
+## Running Linux tools
+
+> Note: That section applies only to MacOS, ignore it on Linux and Windows.
+
+macOS userland (BSD `base64`, `sed`, `date`, ...) differs from the Linux
+userland. Never verify Linux behaviour by running it on the
+Mac - use the local Ubuntu container machine instead.
+
+- Check state: `container machine list`
+- Run a command: `container machine run -n <machine_name> -- <command> [args]`
+- Run a command on the default machine: `container machine run -- <command> [args]`
+- Stop it: `container machine stop <machine_name>`
+
+The `--` separator is required when the command takes arguments; without it the
+call fails with `Operation not supported by device`. A stopped machine is
+started automatically by `run`. The current working directory is available
+inside the machine, so put throwaway scripts under `tmp/` (gitignored) and run
+them as `container machine run -n ubuntu -- bash tmp/check.sh` - the scratchpad
+directory is not visible there.
+
+Verify on Linux whenever a command's behaviour is platform-sensitive, and say
+which platform any test result came from.