fix: deduplicate issues across paginated responses

[wangedmund77-cmyk]

/claim #1

Summary

• Deduplicates issues across paginated GitHub responses using the issue API URL, falling back to ID when needed.
• Keeps pagination driven by resp.NextPage while preserving the since query across pages.
• Adds a regression test for duplicate issues returned on different pages and fixes test server setup so handlers can safely build Link headers.

Verification

• Not run locally: Go toolchain is not installed in this environment (go: command not found).
• Static review confirms the new test covers two paginated requests, since preservation, and three unique issues from four returned rows.

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