fix(reopen): cap issue-events pagination at 10 pages and cover scoring branches

[JSONbored]

Summary

Two follow-up fixes for recently merged PRs:

Pagination cap (#1185 follow-up): getLastCloserLogin walked all issue-events pages with no upper bound — on a PR with thousands of events this could exhaust the GitHub API rate limit and stall the queue processor. Cap at 10 pages (1 000 events), which covers any real PR timeline. Returns the best lastCloser found if the cap is reached. Adds a test asserting the cap fires and page 11 is never requested.

Scoring coverage (#1192 follow-up): Four branches in fetchUpstreamRefSha and the fail-closed guard were uncovered:

• Catch path: SHA lookup throws (network error) → returns null → mutable-ref fallback
• Non-string SHA: endpoint returns { sha: 42 } → typeof guard → null → mutable-ref fallback
• Empty SHA: endpoint returns { sha: "" } → length guard → null → mutable-ref fallback
• Prior-fallback guard: when the last-good snapshot is itself a "fallback", the freeze path must NOT fire — a fresh fallback is persisted instead

Scope

• src/github/pr-actions.ts — add page <= 10 bound + post-loop return
• test/unit/github-pr-actions.test.ts — cap-exit test
• test/unit/scoring.test.ts — 4 new branch-coverage tests

Validation

• npm run test:ci — all 3 734 tests pass

Safety

• The pagination cap is fail-safe: the function already returns null on error and the reopen guard fails open, so the worst case is the caller treats an unknown closer as non-maintainer (allows the reopen).
• No auth or CORS changes.

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.80%. Comparing base (1bc3ee6) to head (a433a94).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1193   +/-   ##
=======================================
  Coverage   94.80%   94.80%           
=======================================
  Files         157      157           
  Lines       19090    19091    +1     
  Branches     6912     6912           
=======================================
+ Hits        18098    18099    +1     
  Misses        399      399           
  Partials      593      593           

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