fix: automate git credentials setup during container initialization

- Configure git credentials at the beginning of init-repo.sh
- Set up git user name/email with defaults or from environment variables
- Create .git-credentials file with proper permissions (600)
- Support multiple GitHub credential formats for better compatibility
- Create symlinks to handle different credential file locations
- Add test script to verify credentials configuration
- Update documentation with new environment variables

This fixes issues where containers fail due to missing .git-credentials
by ensuring credentials are properly configured before any git operations.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

scripts/test-git-credentials.sh

@@ -0,0 +1,46 @@
+#!/bin/bash
+# Test script to verify git credentials configuration
+
+set -e
+
+echo "=== Git Credentials Test ==="
+
+# Check git configuration
+echo "1. Git user configuration:"
+git config --global user.name || echo "  ❌ user.name not set"
+git config --global user.email || echo "  ❌ user.email not set"
+
+echo ""
+echo "2. Git credential helper:"
+git config --global credential.helper || echo "  ❌ credential.helper not set"
+
+echo ""
+echo "3. Credentials file locations:"
+CREDENTIALS_FILE="/config/userdata/.git-credentials"
+if [ -f "$CREDENTIALS_FILE" ]; then
+    echo "  ✓ $CREDENTIALS_FILE exists"
+    echo "  Permissions: $(stat -c %a $CREDENTIALS_FILE)"
+    echo "  Lines in file: $(wc -l < $CREDENTIALS_FILE)"
+else
+    echo "  ❌ $CREDENTIALS_FILE does not exist"
+fi
+
+if [ -f "$HOME/.git-credentials" ]; then
+    if [ -L "$HOME/.git-credentials" ]; then
+        echo "  ✓ $HOME/.git-credentials is a symlink to $(readlink -f $HOME/.git-credentials)"
+    else
+        echo "  ✓ $HOME/.git-credentials exists (not a symlink)"
+    fi
+else
+    echo "  ❌ $HOME/.git-credentials does not exist"
+fi
+
+echo ""
+echo "4. Environment check:"
+echo "  HOME=$HOME"
+echo "  GITHUB_TOKEN=${GITHUB_TOKEN:+[SET]}"
+echo "  GIT_USER_NAME=${GIT_USER_NAME:-[NOT SET]}"
+echo "  GIT_USER_EMAIL=${GIT_USER_EMAIL:-[NOT SET]}"
+
+echo ""
+echo "=== Test Complete ==="