fix(macos): shorten the podman runtime dir to fit sun_path

podman derives conmon's attach socket as
`$XDG_RUNTIME_DIR/libpod/tmp/socket/<64-hex-id>/attach`. With the
descriptive `/tmp/bot-bottle-podman-run` that path is 116 bytes, past
the 108-byte sun_path limit, so every attached `docker run` failed with
"unable to upgrade to tcp, received 500" — while image pulls and the
service itself worked, which is what made it look like a compat-API bug
rather than a path-length one.

Found on the macOS host: quay.io pulled fine and then no container
could be started. Test pins the budget so a more readable name cannot
silently reintroduce it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/backend/macos_container/nested-containers-init.sh

@@ -32,7 +32,9 @@ for device in /dev/fuse /dev/net/tun; do
     }
 done
 
-export XDG_RUNTIME_DIR="${XDG_RUNTIME_DIR:-/tmp/bot-bottle-podman-run}"
+# Short by necessity, not by accident: conmon's attach socket lives under
+# this directory and must fit in a 108-byte sun_path. See nested_containers.py.
+export XDG_RUNTIME_DIR="${XDG_RUNTIME_DIR:-/tmp/bbp}"
 config="$HOME/.config/containers"
 mkdir -p "$XDG_RUNTIME_DIR" "$config"
 chmod 700 "$XDG_RUNTIME_DIR"