fix: correct license field syntax in pyproject.toml

Replace deprecated 'license-files' with proper license field syntax.
Use 'license = {text = "SPDX-ID"}' for standard licenses and
'license = {file = "LICENSE"}' for proprietary licenses.

This fixes the RUF200 parsing error in generated projects.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: dguido

{{cookiecutter.project_slug}}/pyproject.toml

@@ -3,14 +3,12 @@ name = "{{ cookiecutter.project_slug }}"
 dynamic = ["version"]
 description = "{{ cookiecutter.project_description }}"
 readme = "README.md"
-license-files = ["LICENSE"]
-
 {%- if cookiecutter.license == "Apache 2.0" %}
-license = "Apache-2.0"
+license = {text = "Apache-2.0"}
 {%- elif cookiecutter.license == "AGPL v3" %}
-license = "AGPL-3.0-or-later"
+license = {text = "AGPL-3.0-or-later"}
 {%- elif cookiecutter.license == "Proprietary" %}
-license = "LicenseRef-Proprietary-License"
+license = {file = "LICENSE"}
 {%- endif %}
 
 authors = [