A pure read says who opened the last hour's pulls: per author, how many, and how many of those serve a flow issue

Flow work for #5930. On 2026-10-26, 31 feature pulls opened in an hour during the freeze, and only the steward's probe showed it.

Done when:

[ ] A new src/flow/openers.ts exports readOpeners(db, now, publicOnly): per author, the pulls opened in the last hour, most first. Each pull counts as flow when its linked issue is one of #5930, #6217 or #5994, or waits on one of them in the dependency graph.
[ ] It uses the same publicOnly rule as readVitals and at most two SQL reads.
[ ] Tests: two authors are counted and ordered; a pull whose issue waits on #5930 is flow; a pull opened 61 minutes ago is left out; publicOnly hides private repositories.
[ ] Nothing reads it yet.

Files: src/flow/openers.ts, a new test/flow-openers.test.ts.

A pure read says who opened the last hour's pulls: per author, how many, and how many of those serve a flow issue #6443