fix: preserve repository identity in PR linked-issue readiness checks

Actual Behavior

Run python .github/scripts/check_pr_description.py on a PR whose Issue Number section contains Fixes OpenHands/OpenHands#17236 (SDK PR #4931). The checker reports no linked issue even though the linked issue exists and has ready-for-dev. A qualified reference in the Issue Number section can also be mistaken for a local issue with the same number.

Desired Behavior

Extend the existing linked-issue parser and readiness check to preserve the repository in GitHub shorthand references. Reuse the same parser for linked-PR refresh so foreign issues cannot accidentally trigger checks for a same-number local issue. Do not create duplicate tracking issues to satisfy the validator.

Acceptance Criteria

• Unqualified #123 continues to resolve in the PR repository.
• owner/repo#123 after a closing keyword or in Issue Number resolves in the named repository.
• Deduplication uses repository and number, and missing/unready foreign references do not pass as local references.
• Existing linked-PR refresh retains local behavior without confusing foreign numbers.
• Focused parser/validation/refresh tests cover local and cross-repository references. Ready-for-dev when these criteria pass the repository readiness check.

Read .github/scripts/check_pr_description.py and trace the linked-issue parser, readiness validation, and linked-PR refresh. Add focused tests covering local and owner/repo#number references, repository-aware deduplication, and missing or unready foreign issues; done means the acceptance criteria pass the repository readiness check.