fix: increase closingIssuesReferences limit to 100 and warn on truncation in _PR_TIMELINE_QUERY

[Tet-9]

Fixes #885

Problem

_PR_TIMELINE_QUERY requested closingIssuesReferences(first: 20).
A PR closing more than 20 issues has its 21st-and-later closing
references silently truncated. find_solver_from_cross_references
filters merged PRs by issue_number in p.get('closing_numbers', []),
so any issue at position 21+ returns no matches and the validator
records solver_lookup_failed=True — the miner loses credit for a
PR they legitimately merged.

Fix

Two changes to github_api_tools.py:

1. Increased the limit from first: 20 to first: 100 (GitHub's
   per-field maximum for closingIssuesReferences). This eliminates
   the truncation for the overwhelming majority of real-world PRs.

2. Added pageInfo { hasNextPage } to the query and a
   bt.logging.warning when hasNextPage is True. PRs closing
   more than 100 issues are vanishingly rare, but operators can now
   correlate the warning with a missed solver attribution instead of
   debugging silently wrong results.

Changes

• gittensor/utils/github_api_tools.py — first: 20 → first: 100,
  added pageInfo { hasNextPage }, added truncation warning

[anderdc]

In practice we only want PRs that solve one issue — if a PR closes multiple issues, either the original issue wasn't scoped right or the PR is expanding scope. 20 is already very high given that. Closing.