ci: remove PR issue acceptance check - #4543

Summary

A check that failed when linked issues were not accepted was found to be quite verbose and caused confusition with submitters even though the check was not required.

For this reason, this PR removes the check that checks whether linked issues are all accepted. The additions to the review skills to call out unaccepted issues from #4340 are still kept. This should call this out when reviewing PRs.

Related Issue

No issue required: this is a localized CI maintenance change requested directly by a repository administrator, who explicitly requested no new issue.

Changes

• Delete the issue acceptance workflow, checker script, and checker tests.
• Remove their CODEOWNERS entries and workflow security suppression.
• Update contributor notes to describe manual acceptance review.
• Leave skill files unchanged.

Testing

• mise run pre-commit passes.
• Remaining security configuration YAML parses; no stale workflow or checker references remain in CI or contributor docs.
• git diff --check passes.
• mise run ci attempted; Rust test linking fails because the local linker cannot find the Z3 library (ld: library 'z3' not found).
• Unit and E2E tests are not applicable to this workflow condition and documentation change.

Checklist

• Follows Conventional Commits.
• Commit signed off for DCO.
• Architecture documentation updates are not applicable.