Nothing in this suite evaluated nix/agent-modules/ at all: every fixture
was a host, so a typo in a rendered container config surfaced on a real
deploy and nowhere else. This adds an `agent` constructor beside `hive`,
off the same `nixosModules.agent-base` the meta flake hands a container.
It also adds the suite's first two-hive fixture. Every existing one
declares `swarm.hives.h1` alone, so a per-hive arm written against those
passes on a hardcoded literal — which is exactly what the new per-hive
logs pipeline needed covering.
Eight cases, each paired with the control that makes it mean something:
the absence arms with a presence half, the per-hive arm with a roster
length check, because `lib.all` over an empty roster holds vacuously.
Each was then shown to fail. Eight mutations across the three files —
`directory` back to the runtime default, a pipeline naming no receiver,
an exporter aimed at a loopback literal, an exporter name that stops
reading `protocol`, a missing hive-tier logs pipeline, the metrics-only
processor inside it, a per-hive pipeline hardcoded to one hive, and logs
pipelines exporting to the metrics store — all caught, none survived,
none skipped, each run's baseline green.
Part of #3940.
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