fix(mirror): reconcile stale solved issue links

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Summary

• Reconcile issues.solved_by_pr whenever PR metadata is refreshed.
• Clear stale solver links that still point to the refreshed PR after closing references change or the PR is no longer merged.
• Reapply current closing-reference links for merged PRs.

What changed

• handlePrMetadata() now records the previous closing issue set before fetching fresh metadata.
• The refreshed closing issue set is normalized before storage and reconciliation.
• Stale links are cleared only when solved_by_pr still points at the refreshed PR, so links already moved to another PR are preserved.

Why

solved_by_pr was previously write-only on merged PR metadata refreshes. If a merged PR body was edited to remove or change a closing reference, the old issue row could remain permanently linked to that PR and downstream scoring/API views could attribute the issue to the wrong solver.

Fixes #59

Validation

• npm ci
• npm run build
• npm run lint
• npm run format:check
• git diff --check
• Codex Security diff scan: no findings
• CodeRabbit uncommitted review: 0 findings

Notes

• Scope is limited to PR metadata reconciliation.
• No webhook delivery changes, DB migrations, GraphQL rewrites, or periodic reconciliation jobs are included.