doctor: complete runtime check documentation sprint

Signed-off-by: master <>

docs/doctor/articles/servicegraph/servicegraph-valkey.md

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+---
+checkId: check.servicegraph.valkey
+plugin: stellaops.doctor.servicegraph
+severity: warn
+tags: [servicegraph, valkey, redis, cache]
+---
+# Valkey/Redis Connectivity
+
+## What It Checks
+Reads `Valkey:ConnectionString`, `Redis:ConnectionString`, `ConnectionStrings:Valkey`, or `ConnectionStrings:Redis`, parses the host and port, and opens a TCP connection.
+
+The check skips when no cache connection string is configured and fails when parsing fails or the target cannot be reached.
+
+## Why It Matters
+Cache unavailability affects queue coordination, state caching, and latency-sensitive platform features. A malformed connection string is also an early warning that the environment is not wired correctly.
+
+## Common Causes
+- The cache connection string is missing, malformed, or still points to a previous environment
+- The Valkey/Redis service is not running
+- Container networking or DNS is broken
+
+## How to Fix
+
+### Docker Compose
+```yaml
+services:
+  doctor-web:
+    environment:
+      Valkey__ConnectionString: valkey:6379,password=${STELLAOPS_VALKEY_PASSWORD}
+```
+
+```bash
+docker compose -f devops/compose/docker-compose.stella-ops.yml ps valkey
+docker compose -f devops/compose/docker-compose.stella-ops.yml exec doctor-web sh -lc "nc -zv valkey 6379"
+```
+
+### Bare Metal / systemd
+```bash
+redis-cli -h <valkey-host> -p 6379 ping
+```
+
+### Kubernetes / Helm
+Use a cluster-local service name in the connection string and verify the port exposed by the StatefulSet or Service.
+
+## Verification
+```bash
+stella doctor --check check.servicegraph.valkey
+```
+
+## Related Checks
+- `check.servicegraph.mq` - both checks validate internal service-network connectivity
+- `check.servicegraph.endpoints` - broad service discovery issues usually affect cache endpoints too