feat: add useCoursenameAsPrefix option to customize repository naming convention

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Pull request overview

Adds a configuration option to optionally prefix generated repository names with the course name, centralizing repository naming logic in AssignmentConfig and updating GitLab/Git operations and documentation accordingly.

Changes:

• Introduce UseCoursenameAsPrefix and new helper methods (RepoBaseName, RepoNameWithSuffix, RepoNameForStudent, RepoNameForGroup) to standardize repo naming.
• Update GitLab operations (generate/update/archive/delete/protect/setaccess) and git clone/path construction to use the new naming helpers.
• Document the new configuration option and naming behavior in README.md, and display it in config.Show() output.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
gitlab/update.go	Use centralized repo naming when resolving project paths for updates.
gitlab/setaccess.go	Use centralized repo naming when resolving project paths for access changes.
gitlab/protect.go	Use centralized repo naming when resolving project paths for branch protection.
gitlab/generate.go	Use centralized repo naming when generating per-student/per-group projects.
gitlab/delete.go	Use centralized repo naming when deleting projects.
gitlab/archive.go	Use centralized repo naming when resolving project paths for archive/unarchive.
git/clone.go	Switch clone URL and local path construction to use new base-name helpers.
config/urls.go	Print URLs using the new repo naming helpers.
config/types.go	Add UseCoursenameAsPrefix to assignment config structure.
config/show.go	Display the new option in config output.
config/assignment.go	Populate the option from viper and add naming helper methods.
README.md	Document useCoursenameAsPrefix and resulting naming conventions.

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