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>

configure-container.sh

@@ -0,0 +1,46 @@
+#!/bin/sh
+set -eu
+
+script_dir=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+helper_path="$script_dir/git-credential-hostproxy"
+scope='global'
+target_repo=$(pwd)
+
+while [ "$#" -gt 0 ]; do
+  case "$1" in
+    --global)
+      scope='global'
+      ;;
+    --local)
+      scope='local'
+      ;;
+    --repo)
+      shift
+      if [ "$#" -eq 0 ]; then
+        printf 'Missing value for --repo\n' >&2
+        exit 1
+      fi
+      target_repo="$1"
+      ;;
+    *)
+      printf 'Usage: %s [--global|--local] [--repo PATH]\n' "$0" >&2
+      exit 1
+      ;;
+  esac
+  shift
+done
+
+if [ "$scope" = 'global' ]; then
+  git config --global --replace-all credential.helper ''
+  git config --global --add credential.helper "$helper_path"
+  git config --global credential.useHttpPath true
+  printf 'Configured global Git credential helper: %s\n' "$helper_path"
+else
+  git -C "$target_repo" config --local --replace-all credential.helper ''
+  git -C "$target_repo" config --local --add credential.helper "$helper_path"
+  git -C "$target_repo" config --local credential.useHttpPath true
+  printf 'Configured local Git credential helper for %s\n' "$target_repo"
+fi
+
+printf 'Proxy URL default: %s\n' "${GIT_CRED_PROXY_URL:-http://host.docker.internal:18765}"
+printf 'Protocol filter is configured on the host side\n'