Adds a new 'dns' semantic monitoring kind that probes whether a DNS name
resolves. Uses net.LookupNS (NS records) with fallback to net.LookupHost
(A/AAAA). Supports an explicit server config for split-horizon resolution.
Changes:
- seeds/ontology.yaml: dns-zone monitoring: none → [dns] (was deferred
since 2026-06 with a comment 'no dns checker exists yet')
- seeds/inventory.yaml: host:netbird-vps monitoring: [http] (was none;
VPS was invisible for 7 days during the 2026-07-29 outage)
- internal/checkdefaults/defaults.go: add KindDNS, buildKind case for 'dns'
that creates a check_def at 5-minute intervals
- internal/scheduler/scheduler.go: add checkDNS probe + wire in executeCheck
The DNS checker catches stale/unreachable zones (e.g. matrix.hubris.network
pointing to a dead VPS IP). The VPS HTTP check probes the public endpoint
every 60s, closing the 7-day monitoring gap.
dtoro
0920c4cb6d