EVALS_CONCURRENCY was overloaded: the legacy bun-test path used it as
--max-concurrency (default 15) while the sharded runner read it as the
process count — exporting the legacy value gave 15 concurrent Bun
processes each spawning claude (the 429 storm). Now: EVALS_JOBS = shard
processes (default 4); EVALS_CONCURRENCY = bun --max-concurrency inside
a shard (default 4, explicit in shard args — omitting it made
within-shard parallelism silently differ from the legacy path). Stale
49/59 header math replaced with the live-count rule.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>