ADR 0189: the store keeps what the records name, and a maintenance step holds its writers still

Issue 108: the artifact store has never collected anything. Fifty-three
repositories on the machine that serves everything else, and the only outcome
of leaving it is a full disk reported as somebody else's failure.

The mesh decides what may go — from its own build records, so it never names
a digest it did not put there — and the store reclaims the bytes in a nightly
window with its server held still. Deletion on the one door takes nothing a
push did not already have.

Designs 18 and 20 amended; issue 108 resolved.

Also issue 202, found running the controller's suite: a module whose required
setting nobody set is left out of the machine in silence, and dnsmasq became
that module this morning.

02-DECISIONS/README.md

@@ -189,6 +189,7 @@ python3 00-META/checks/index.py            fail if stale
 - **0186** — [A ban list never holds a neighbour, and the mesh's own bans are its own wherever they hang](0186-a-ban-list-never-holds-a-neighbour.md)
 - **0187** — [A dead tracker is not the machine's failure](0187-a-dead-tracker-is-not-the-machines-failure.md)
 - **0188** — [A provider declares what it derives for each consumer, and the mesh tells both ends](0188-a-provider-declares-what-it-derives-for-each-consumer.md)
+- **0189** — [The store keeps what the records name, and a maintenance step holds its writers still](0189-the-store-keeps-what-the-records-name.md)
 
 ### Its tiers, from the bottom up