A tick starts one builder run per free slot, not one issue per tick #8729

Left open from #8473 (pull #8509, 6712fb23): the picker now skips an issue the dispatcher refuses, but a tick still starts one issue per call, so three free slots fill over three ticks.

Done looks like:

• [x] One tick starts as many startable issues as there are free builder slots (the builder seat's number from #7668), each through the same freshPullFor test the dispatcher uses.
• [x] A held issue (open pull, landed, hot file) never takes a slot; the next startable one does, in the same tick.
• [x] A test: three free slots and four startable issues, one with a fresh pull; one tick starts exactly three runs on the other three.
• [x] The tick line still counts the held one under held.

Files: src/crew/autonomy.ts, src/crew/dispatch.ts, test/crew-held.test.ts Risks: spend. The day's ceiling and subscription headroom must be checked before each start, not once per tick.

Reviewed pull #8796 at its head: ticked One tick starts as many startable issues as there are free builder slots (the builder seat's number from #7668), each through the same freshPullFor test the dispatcher uses.; A held issue (open pull, landed, hot file) never takes a slot; the next startable one does, in the same tick.; A test: three free slots and four startable issues, one with a fresh pull; one tick starts exactly three runs on the other three.; The tick line still counts the held one under held..