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

This PR updates the external/dependabot reviewer-assignment GitHub Actions workflow to select a reviewer deterministically based on the PR number, avoiding clustering that can happen with random selection for consecutive Dependabot PRs.

Changes:

• Replace Math.random() reviewer selection with pull_request.number % reviewers.length.
• Update workflow/step wording to reflect deterministic selection and document the rationale.

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