Generates a new repo from mathias/template-go-web via Gitea's generate
API, then substitutes __PROJECT_NAME__ and __MODULE_PATH__ placeholders
in six known files (best-effort, partial failure surfaced in result).
Validates name regex, allowlist, template flag, and destination
non-existence before generating. Adds Template field to gitea.Repo.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
