Add container Git helper integration

Wire Git inside containers to the host proxy so HTTPS remotes can reuse host credentials automatically.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

git-credential-hostproxy

@@ -0,0 +1,19 @@
+#!/bin/sh
+set -eu
+
+script_dir=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+
+if [ -n "${GIT_CRED_PROXY_RUNTIME:-}" ]; then
+  exec "$GIT_CRED_PROXY_RUNTIME" "$script_dir/helper.mjs" "$@"
+fi
+
+if command -v bun >/dev/null 2>&1; then
+  exec bun "$script_dir/helper.mjs" "$@"
+fi
+
+if command -v node >/dev/null 2>&1; then
+  exec node "$script_dir/helper.mjs" "$@"
+fi
+
+printf 'Either bun or node is required to run git-credential-hostproxy\n' >&2
+exit 1