fix: use last:50 instead of first:50 for solver cross-reference lookup (#1017)

[alpurkan17]

Summary

• Change first:50 to last:50 in GraphQL query for solver repository lookup
• This returns the most recent 50 PRs, where the solver's active PRs live

Root Cause

Solver cross-reference lookup uses first:50 to fetch PRs, which returns the OLDEST PRs first. The solver's most recent PR (the one affecting current scoring) is likely at the end of the list — often beyond the 50-item window, so it's missed entirely.

Impact

Solver cross-reference resolution misses recent PRs, leading to stale solver data and incorrect scoring of solver contributions.

Why

Solvers are scored on recent PRs. first:50 returns oldest PRs — wrong direction for relevance

Related Issues

Fixes #1017

Test plan

• ruff check
• ruff format --check
• pyright
• pytest tests/utils/test_solver_lookup.py::test_solver_lookup_last_50 -q

How to verify

1. Run the solver lookup test to confirm last:50 returns recent PRs
2. Verify the GraphQL query uses last:50 not first:50

Edge cases considered

• Solver repository has <50 PRs total (last:50 works same as first:50)
• Repository with only old PRs and no recent activity
• Private solver repositories (additional auth needed)

Post-merge verification

• Check solver cross-reference now finds recent PRs correctly
• Verify solver scoring improved for active solvers

[anderdc]

Duplicate of #1018 which is better scoped/implemented. Closing.