Add git user configuration support

Add required git user configuration to enable commits from within the container.
This change ensures proper attribution of git commits made inside the container by:

- Adding GIT_USER_NAME and GIT_USER_EMAIL to .env.example
- Configuring git user globally during Docker build
- Adding documentation for git configuration requirements
- Updating README with clearer setup instructions and requirements

The configuration is now required as part of the initial setup to prevent
issues with unattributed commits when using git inside the container.

scripts/startup.sh

@@ -34,6 +34,13 @@ else
     echo "No Twilio credentials found - SMS notifications disabled"
 fi
 
+# Configure git for the mounted workspace
+if [ -n "$GIT_USER_NAME" ] && [ -n "$GIT_USER_EMAIL" ]; then
+    echo "✓ Configuring git: $GIT_USER_NAME <$GIT_USER_EMAIL>"
+    git config --global user.name "$GIT_USER_NAME"
+    git config --global user.email "$GIT_USER_EMAIL"
+fi
+
 # Start Claude Code with permissions bypass
 echo "Starting Claude Code..."
 exec claude --dangerously-skip-permissions "$@"