Fix pagination loop termination with Since filter

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Fixes #1

Problem

The pagination loop when fetching issues with the Since filter can fail in several edge cases:

1. No nil check on resp before accessing resp.NextPage (can panic)
2. Duplicate issues can be fetched across pages
3. No context cancellation check between pages

Fix

In pkg/github/client.go:

• Added nil response check: if resp == nil || resp.NextPage == 0
• Added deduplication using a seen map keyed by issue ID
• Added context cancellation check between pages
• The Since parameter is already preserved across pages since opt.Since is set once and never modified

In pkg/github/client_test.go:

• Added TestFetchIssues_DuplicatePrevention test that verifies duplicate issues across pages are filtered out

/claim #1

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