API repository creation: apply declared rulesets instead of copying template rulesets

[hdamker]

What type of PR is this?

• enhancement/feature
• repository management

What this PR does / why we need it:

The API repository creation workflow no longer copies the rulesets of Template_API_Repository. It checks the new repository's entry in config/repositories.yaml before creating anything (dry run included), applies the declared rulesets of the api-repository ruleset class with the repository-config tool after template cleanup, and fails if a plan for the new repository still shows a difference. Adds a check-entry command to the tool, and plan/apply with --repos now look up each named repository directly, since a just-created repository may not yet appear in the organisation listing.

• New repositories get exactly the declared rulesets, not whatever the template carries
• A missing or inconsistent registry entry stops the run before the repository exists

Creating a repository now needs its registry entry (ruleset_class: api-repository, and single_codeowner: true for a single initial codeowner) merged by PR first.

Which issue(s) this PR fixes:

Part of #310

Special notes for reviewers:

Based on #311; review after it is merged (the diff shows both until then).

Changelog input

API repository creation applies the declared rulesets and requires a registry entry for the new repository

Additional documentation

workflows/api-repository-creation/docs/README.md updated.