Fix pagination loop termination with since filter

[Muyutxd1]

Fixes #1
/claim #1

Summary

• prevent pagination loops when the GitHub API returns a non-advancing NextPage
• preserve Since pagination behavior through the existing options flow
• de-duplicate issues returned across paginated responses by GitHub issue ID

Tests

• Added regression coverage for non-advancing pagination with since
• Added regression coverage for duplicate issues across pages
• Not run locally: Go toolchain is not installed in this environment

[opirebot]

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