Fix GitHub issue pagination with since filter

[felixvippp-ai]

Closes #1

Summary

• Switch the issue fetcher to the correct go-github ListByRepo API.
• Continue pagination from the response NextPage value so short pages do not stop the loop early and non-sequential page numbers are respected.
• Preserve Since-filtered pagination coverage and skip duplicate issues across pages using stable issue identifiers.
• Add go.sum so the module can be tested reproducibly.

Validation

• go test ./...

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