issue_probe's merged-PR argument becomes a spec list — `PR` or `PR@<iso>`
— so a probe can state merge order; the bare form keeps every existing call
site literal.
The direct-drive cases cover crew#176's shape (the lower number merged
later), agreeing orders, interleaved issues, the mergedAt tie broken by
highest PR number under both input orders, and the empty answer. The
end-to-end probe is crew#321's: a marker already standing for the
later-merged, lower-numbered PR must suppress the transition, which
selecting by number could never do.
Two static pins keep the request count honest — the sweep issues exactly two
GraphQL queries, with mergedAt selected on the merged-PR node it already
fetched.
Refs #242
