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annotated as superseded; its applied SQL is untouched.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.

docs/phase-7-runbook.md

@@ -74,7 +74,7 @@ Run it as a container on the compose network with a network alias of
 `OLLAMA_BASE_URL` at it via a throwaway compose override:
 
 ```sh
-docker run -d --rm --name cairnobs-mock-ollama --network sentry_default --network-alias ollama \
+docker run -d --rm --name cairnobs-mock-ollama --network cairnobs_default --network-alias ollama \
   -v "$(pwd)/hack/mock-ollama:/src" -w /src golang:1.25-alpine \
   sh -c "go build -o /tmp/mock-ollama . && /tmp/mock-ollama"
 
@@ -188,7 +188,7 @@ cd api && go build ./... && go vet ./... && go test ./...
 cd enterprise && go build ./... && go vet ./... && go test ./...
 
 # live-Postgres audit tests specifically, against the real dev stack:
-docker run --rm --network sentry_default -v "$(pwd):/src" -w /src/enterprise \
+docker run --rm --network cairnobs_default -v "$(pwd):/src" -w /src/enterprise \
   -e AUDIT_TEST_POSTGRES_ADDR=metadata-postgres:5432 \
   -e AUDIT_TEST_POSTGRES_PASSWORD=audit-writer-dev-only \
   -e AUDIT_TEST_ADMIN_PASSWORD=cairnobs-dev-only \