Restructure refimpl into go-lang and python subdirectories

Move Go reference implementation to refimpl/go-lang/ and add new
Python reference implementation in refimpl/python/. Update build.sh
with renamed draft and simplified tool paths.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

build.sh

@@ -1,23 +1,12 @@
 #!/bin/bash
 set -e
 
-DRAFT="draft-nennemann-wimse-execution-context-00"
+DRAFT="draft-nennemann-wimse-ect-00"
 DIR="$(cd "$(dirname "$0")" && pwd)"
 
-# Resolve tool paths
-KDRFC="$(find /usr/local/lib/ruby/gems /opt/homebrew -name kdrfc -path "*/bin/*" 2>/dev/null | head -1)"
-KRAMDOWN="$(find /usr/local/lib/ruby/gems /opt/homebrew -name kramdown-rfc2629 -path "*/bin/*" 2>/dev/null | head -1)"
-XML2RFC="$(find "$HOME/Library/Python" /usr/local /opt/homebrew -name xml2rfc -path "*/bin/*" 2>/dev/null | head -1)"
-
-if [ -z "$KRAMDOWN" ]; then
-  echo "Error: kramdown-rfc2629 not found. Install with: gem install kramdown-rfc" >&2
-  exit 1
-fi
-
-if [ -z "$XML2RFC" ]; then
-  echo "Error: xml2rfc not found. Install with: pip install xml2rfc" >&2
-  exit 1
-fi
+# Tool paths
+KRAMDOWN="/usr/local/lib/ruby/gems/3.4.0/bin/kramdown-rfc2629"
+XML2RFC="/Users/christian/Library/Python/3.9/bin/xml2rfc"
 
 export PYTHONWARNINGS="ignore::UserWarning"