fix(postCreate): fail fast with a clear message when .git is missing

If the user opens the devcontainer before running `git init`, uv sync
fails opaquely because setuptools-scm can't compute a version, and
pre-commit can't install hooks into a non-existent .git/hooks. VS
Code's wrapper then hides the real stderr behind a generic
"postCreateCommand failed" exit-1.

postCreate.sh now detects a missing .git directory up front, prints
a clear explanation to stderr, and exits 1. The user is told exactly
what to run on the host to recover. Failing fast beats silently
bootstrapping a half-broken environment.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: gilesknap

template/.devcontainer/postCreate.sh.jinja

@@ -4,6 +4,33 @@ set -euo pipefail
 # Install Claude Code CLI
 curl -fsSL https://claude.ai/install.sh | bash
 {% endif %}
+# Refuse to continue without a git repo. setuptools-scm needs git
+# tags to compute the package version, and pre-commit installs its
+# hooks into .git/hooks — both fail with cryptic errors that VS Code
+# then hides behind a generic "postCreateCommand failed" message.
+# Better to stop here with a clear explanation.
+if [ ! -d .git ]; then
+    cat >&2 <<'EOF'
+
+================================================================
+ERROR: This directory is not a git repository.
+
+setuptools-scm needs git history to compute the package version,
+and pre-commit installs its hooks into .git/hooks. Neither will
+work without a git repo.
+
+To fix this, run on the host (outside the devcontainer):
+
+    git init -b main && git add . && git commit -m 'Initial commit'
+
+then rebuild the devcontainer.
+
+================================================================
+
+EOF
+    exit 1
+fi
+
 # Install Python dependencies and pre-commit hooks
 uv venv --clear
 uv sync