feat: add template bootstrapping for project create

Adds --template/-t flag to `gitnow project create` that copies files
from a template directory into new projects. Templates are discovered
from ~/.gitnow/templates/ (configurable via settings.project.templates_directory).
Includes a default template with a SPEC.md scaffold.

templates/default/SPEC.md

@@ -0,0 +1,17 @@
+# Project Spec
+
+## Overview
+
+<!-- Describe the purpose and goals of this project -->
+
+## Repositories
+
+<!-- List the repositories included and their roles -->
+
+## Architecture
+
+<!-- Describe how the repositories relate to each other -->
+
+## Getting Started
+
+<!-- Steps to get up and running after cloning -->