Assign external-PR reviewers deterministically, not randomly

[noahho]

Replaces the random reviewer pick with a deterministic one keyed on the PR number. The file stays as the source of reviewers.

-const reviewer = reviewers[Math.floor(Math.random() * reviewers.length)];
+const reviewer =
+  reviewers[context.payload.pull_request.number % reviewers.length];

Why

Dependabot opens its PRs as a run of consecutive numbers, and Math.random() happily gives the same person several in a row - one reviewer drew both #368 and #369 in tabpfn-extensions. Modulo makes consecutive numbers structurally distinct.

Measured against the real eligible-PR history (fork + Dependabot PRs, 8 reviewers):

	TabPFN (59 PRs)	tabpfn-extensions (101 PRs)
Back-to-back repeats, modulo	3 of 58	3 of 100
Back-to-back repeats, random	8	13
Consecutive-number pairs landing on the same reviewer	0 of 27	0 of 40

What this is not: a round robin. Long-run counts are about as uneven as random (modulo spread 4-10 across 8 buckets on TabPFN, 8-16 on tabpfn-extensions; random baseline 4-11 and 8-18), because the PR numbers that reach this workflow are sparse - it fires only for forks and Dependabot, and PR numbers share a counter with issues. Equal counts per reviewer would need stored state or GitHub's own team-based review assignment. This fixes the clustering, not the fairness, and the comment in the workflow says so.

Side benefit: it is idempotent. A reopen or ready-for-review re-request now names the same person instead of possibly adding a second reviewer.

Same change is going into the other copy of this workflow (TabPFN and tabpfn-extensions are the only two repos that have it).

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Updates the external/dependabot PR reviewer assignment workflow to choose reviewers deterministically (based on PR number) instead of randomly, reducing clustering for consecutive Dependabot PRs while keeping the reviewer list file as the single source of truth.

Changes:

• Replace Math.random()-based reviewer selection with PR_number % reviewers.length.
• Update workflow step naming and inline rationale/comments to reflect deterministic selection.

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