fix(signals): group contributor open PRs by repo case-insensitively

[galuis116]

Summary

Fixes #1526.

groupByRepo in buildContributorOpenPrMonitor bucketed a contributor's open PRs by the raw pr.repoFullName, while every other repo-name operation in the same function is case-insensitive (the registered-repo set, the open-PR filter, the repo lookup). GitHub repo names are case-insensitive, so case-variant repoFullName values for one repo split into separate groups — under-counting per-repo open PRs, running duplicate detection only within each partial group, and querying listPullRequests once per casing.

What changed

• groupByRepo now keys buckets on pr.repoFullName.toLowerCase(), matching the function's own repo-name handling.
• The consumer loop derives the canonical repoFullName from a bucket's first PR (its original casing) for the case-sensitive per-repo DB queries — the established key-by-lower / keep-original pattern used in contributor-evidence-graph.ts.

Scope

• Conventional Commit title.
• Focused; no unrelated changes.
• Follows CONTRIBUTING.md; no site//CNAME/VitePress.
• Linked issue ([Bug]: contributor open-PR monitor groups PRs by case-sensitive repoFullName, splitting one repo into two #1526).

Validation

• git diff --check
• npm run typecheck
• npm run test:coverage locally (4619 passed); the new grouping lines are covered by a case-variant test, and the existing same-casing tests are unchanged.
• npm run test:workers
• npm audit --audit-level=moderate
• New unit test: two open PRs for one repo under different casings group together and the case-sensitive query never runs against the case-variant.

If any required check was skipped, explain why:

• OpenAPI/types/migrations unchanged — internal grouping only; the monitor's output shape is identical.

Safety

• No secrets, wallets, hotkeys, PATs, trust scores, or private evidence exposed.
• Public text sanitized and low-noise (monitor output sanitizer test still passes).
• No auth/CORS/session change.
• API/OpenAPI/MCP behavior unchanged.
• No UI changes.
• No changelog edit.

Notes

• Confined to src/signals/contributor-open-pr-monitor.ts; mirrors the case-insensitive repo handling already present in the same function and in contributor-evidence-graph.ts.

[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 95.46%. Comparing base (6b1389b) to head (c6ea025).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1527   +/-   ##
=======================================
  Coverage   95.45%   95.46%           
=======================================
  Files         195      195           
  Lines       21121    21123    +2     
  Branches     7637     7637           
=======================================
+ Hits        20161    20165    +4     
  Misses        383      383           
+ Partials      577      575    -2     

Files with missing lines	Coverage Δ
src/signals/contributor-open-pr-monitor.ts	98.11% <100.00%> (+1.95%)	⬆️

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