A test boots the flow-label dispatcher: labelling a held issue `flow` starts its run, that issue only, paid by whoever last assigned it #7512

Intent: release 0.7.283 did not boot because of the flow-label dispatcher (pull #7436). A test that imports and runs src/crew/flow_label.ts would have failed first. No test imports it directly today.

Done:

• [ ] A new test/flow-label-dispatch.test.ts makes an app with "flow only" on, an issue assigned to an agent and held, then adds the flow label. It checks that dispatchFlowLabelled starts exactly one run for that issue (no other held issue starts), and that the run is charged to the last assigner.
• [ ] It also checks that labelling an issue that is not held, or one with no assignee, starts nothing and throws nothing.
• [ ] The test imports the module the way src/main.ts does at boot, so an import-time failure fails it.
• [ ] If any of these fail, fix src/crew/flow_label.ts, and the pull says what was wrong.

Files: a new test, and src/crew/flow_label.ts only if the test finds a fault.