Complete the low-risk half of the Sentry -> Cairn OBS rebrand

Sweeps the references that carry no runtime coupling, and fixes one that
turned out to be a real bug rather than stale branding.

Docker network: sentry_default -> cairnobs_default across 23 runbook and
test-header `docker run` commands. Compose derives the network from the
directory name, so this lands together with renaming the working copy to
cairnobs/ -- the two are only correct as one change.

Stale references corrected: four Dockerfile "repo root (sentry/)"
headers; .env pointing at the long-renamed deploy/helm/sentry/ chart;
five Helm comments describing the topic as sentry.logs.raw when all four
code paths have defaulted to cairnobs.logs.raw for some time; an
absolute /home/john/Projects/sentry/ path in the operator's package doc,
now repo-relative; the hand-written Tenant CRD description in both of
its identical copies, whose Go source already said Cairn OBS.

Migration 0043 repoints the default tenant's data source. 0026 seeded it
with ('sentry', '/var/lib/sentry-search') to match what
api/internal/config then defaulted to; the rebrand later moved those
defaults to "cairnobs" and /var/lib/cairnobs-search without moving the
already-applied row, leaving the default tenant naming a ClickHouse
database nothing writes to. Scoped to the exact stale values so it is a
no-op on any deployment that set them deliberately. 0026's comment is
annotated as superseded; its applied SQL is untouched.

Deliberately not included: the gRPC wire packages (sentry.logs.v1,
sentry.agent.v1) and proto/sentry/ import paths, which cannot change
without a lockstep agent/server upgrade; the Helm chart's
sentry_metadata database and sentry role, which need a real Postgres
migration on existing deployments; and the compliance audit records in
docs/compliance/, which are a dated historical record.

go build, go vet, and go test pass for ingest and deploy/operator.

ingest/Dockerfile

@@ -1,4 +1,4 @@
-# Build context must be the repo root (sentry/), not ingest/, since this
+# Build context must be the repo root (cairnobs/), not ingest/, since this
 # needs both ingest/ and proto/:
 #   docker build -f ingest/Dockerfile -t cairnobs-ingest .