fix(scheduler): derive entity health from all its checks, not the last one

host:strong logged 226 health.changed events in one hour, oscillating
down/healthy while the host was fine throughout. host:hubris did it 126 times.

runCheck wrote entity_status.health on every check completion, so an entity's
health was simply whichever of its checks finished most recently. A host with
six checks reported whichever facet happened to be sampled last, and one
failing probe alternating with five passing ones flapped forever. resolveSignal
forced "healthy" too, a second path by which one passing probe erased another
probe's genuine failure.

On this fleet the trigger is a known false positive: the scheduler's network
vantage point cannot ICMP host:strong, so its ping check fails while every
ssh-script check succeeds. Under last-writer-wins that single probe declared
the whole host down, twice a minute.

Each check now records its own verdict (check_defs.last_health, migration 027)
and the entity's health is the worst across its enabled checks. A failing probe
now degrades the entity honestly and *stably*, without erasing what the other
five report, and health.changed fires only when that aggregate actually moves.
Checks that have never run are ignored rather than counted as unknown, so
adding a check cannot drag a known-good entity down before it has a verdict.

Also declares service:oikos in the seed. The previous commit re-pointed the mcp
ingress at it, but the entity only ever existed in the production database — so
a fresh seed (a new install, or a DR restore) failed on an unresolvable edge.
Caught by seeding an empty database rather than a copy of prod, which is the
only way that class of bug shows up.

Co-Authored-By: Claude <noreply@anthropic.com>

internal/db/queries/operations.sql

@@ -68,7 +68,21 @@ WHERE cd.enabled = true
        OR cd.last_run_at <= now() - make_interval(secs => cd.interval_s));
 
 -- name: MarkCheckRun :exec
-UPDATE check_defs SET last_run_at = now() WHERE entity_id = $1;
+UPDATE check_defs SET last_run_at = now(), last_health = $2 WHERE entity_id = $1;
+
+-- name: WorstHealthForTarget :one
+-- An entity is as healthy as its unhealthiest check. Checks that have not run
+-- yet (last_health IS NULL) are ignored rather than counted as unknown, so a
+-- newly added check does not drag a known-good entity down before it has
+-- produced a verdict.
+SELECT COALESCE(
+  (SELECT last_health FROM check_defs
+   WHERE enabled AND target_id = $1 AND last_health IS NOT NULL
+   ORDER BY CASE last_health
+     WHEN 'down' THEN 0 WHEN 'degraded' THEN 1 WHEN 'stale' THEN 2
+     WHEN 'unknown' THEN 3 ELSE 4 END
+   LIMIT 1),
+  'unknown')::text AS health;
 
 -- name: GetCheckDef :one
 SELECT * FROM check_defs WHERE entity_id = $1;