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>

refimpl/python/pyproject.toml

@@ -0,0 +1,25 @@
+[build-system]
+requires = ["setuptools>=61", "wheel"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "ect-refimpl"
+version = "0.1.0"
+description = "WIMSE Execution Context Tokens (ECT) reference implementation"
+requires-python = ">=3.9"
+license = "BSD-3-Clause"
+dependencies = [
+    "PyJWT>=2.8.0",
+    "cryptography>=42.0.0",
+]
+
+[project.optional-dependencies]
+dev = ["pytest>=7.0", "pytest-cov>=4.0"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+pythonpath = ["."]
+addopts = "--cov=ect --cov-report=term-missing --cov-fail-under=90 -v"
+
+[tool.setuptools.packages.find]
+include = ["ect*"]