Add CODEOWNERS plus issue and pull request templates

[knytcomics-ui]

Problem

There are no issue templates, no PR template, and no CODEOWNERS file in .github/. Given this repo's entire value is the correctness of hand-curated ground-truth data, unstructured issues/PRs make it easy for a contributor to propose a new fixture without the reviewer having an easy checklist to verify (does it validate? is the label justified? is the score rationale sound? per the several other issues in this batch defining what "sound" means).

Requirements

• Add .github/ISSUE_TEMPLATE/fixture-request.yml (structured form: proposed label, address/type, justification) and .github/ISSUE_TEMPLATE/bug-report.yml.
• Add .github/PULL_REQUEST_TEMPLATE.md with a checklist: npm run validate passes locally, new fixtures include notes/rationale, README/roadmap updated if structure changed, XDR fixtures regenerated via the generation script (once that issue lands) rather than hand-edited.
• Add .github/CODEOWNERS designating required reviewers for destinations.json/scores.json changes specifically (ground-truth data deserves tighter review than, say, a README typo fix).

Acceptance Criteria

• Opening a new issue/PR on GitHub shows the structured templates.
• A PR touching destinations.json or scores.json requests review from the CODEOWNERS-designated reviewer(s).

Part of the Gryd-lock organization. This repo supplies the labelled testnet fixtures and stub scores consumed by grydlock-oracle-adapter's StubOracle and evaluated end-to-end by grydlock-research.