A pull serves flow when its issue carries the `flow` label, not only when it is on a hard-coded list #6498

Part of #5930. FLOW_ISSUES in src/flow/openers.ts is [5930, 6217, 5994], numbers of one repository, so the count is wrong everywhere else.

Done:

• [ ] readOpeners counts a pull as flow when the issue it closes, or one it waits on, carries the label flow (case-insensitive), as well as the listed numbers.
• [ ] The query stays one statement; no N+1.
• [ ] Test: a pull closing a flow-labelled issue in a second repository counts as flow; an unlabelled one does not.

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

Before pushing: bun x tsc --noEmit and bun test test/flow-openers.test.ts.