A pure read counts the open pulls from outside the repository's writers, and the oldest one's age

Flow work for #5930 ("the outsider count" among the flow vitals). An outsider's pull waits for a person, so it must be counted, not lost in the pile.

Done when:

[ ] A new src/flow/outsiders.ts exports readOutsiders(db, now, publicOnly): { open, oldestHours } for open, unmerged pulls whose author is neither a writer of the repository nor one of its crew agents.
[ ] It uses the same writer rule the API uses for untrusted, and the same publicOnly rule as readVitals.
[ ] Tests: a writer's pull and a crew pull are left out; an outsider's is counted, with its age; publicOnly hides private repositories.
[ ] Nothing reads it yet.

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

A pure read counts the open pulls from outside the repository's writers, and the oldest one's age #6444