fix: resolve pagination loop termination, API method signature, and issue deduplication with Since filter

[yaqi-2006]

Summary

Fixes issue #1: Fix Pagination Loop Termination When Fetching Repository Issues with 'Since' Filter.

Root Cause

1. pkg/github/client.go called the non-existent method .client.Issues.ListByRepository(...) instead of ListByRepo(...).
2. The pagination loop needed to strictly rely on *github.Response.NextPage (from GitHub's Link header) rather than returned slice lengths, preserving the Since filter across all paginated calls.
3. Concurrent issue updates during multi-page traversal could lead to duplicate issues across pages.

Changes Made

• API Method Correction: Updated invocation to .client.Issues.ListByRepo(ctx, owner, repo, opt).
• Response-Driven Pagination: Handled iteration via
  esp.NextPage with defensive loop protection (�isitedPages).
• Deduplication: Added ID-based issue deduplication (seenIDs) to prevent duplicate items from being appended.
• Nil Safety: Added guards for nil client and response references.
• Comprehensive Unit Tests:
  • TestFetchIssues_Pagination: Verifies standard multi-page pagination.
  • TestFetchIssues_EmptyPageWithNext: Handles empty page with a next link.
  • TestFetchIssues_Deduplication: Verifies deduplication across pages.
  • TestFetchIssues_SincePreservedAcrossPages: Verifies Since filter preservation across requests.
  • TestFetchIssues_RateLimit: Tests 403 Rate Limit error propagation.
  • TestFetchIssues_ContextCanceled: Tests context cancellation handling.
  • TestFetchIssues_NilClient: Tests nil client error handling.

Verification

• go test -v -race ./... -> All 7 tests passed (PASS).
• go build -v ./... -> Successfully compiled.

Closes #1

[yaqi-2006]

@opire-dev claim #1