build: split VERSION into userspace/ and kernel/

Move the existing VERSION (userspace 17.2) to userspace/VERSION and add
kernel/VERSION (6.18) matching the linux submodule. Update Dockerfile
COPY path accordingly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: greatgitsby

kernel/VERSION

@@ -0,0 +1 @@
+6.18

tools/build/Dockerfile

@@ -236,7 +236,7 @@ COPY ./userspace/readonly_setup.sh /tmp/vamos/
 RUN /tmp/vamos/readonly_setup.sh
 
 # Version
-COPY VERSION /VERSION
+COPY userspace/VERSION /VERSION
 
 # TODO: Remove Qualcomm GPU blobs by rebuilding pyray/raylib against Mesa
 # Currently raylib is compiled directly against libEGL_adreno.so/libGLESv2_adreno.so