chore(dashboards): rename cerberus-self.json to cerberus.json and drop subtitle

[tsouza]

Summary

User feedback: compose-stack dashboards should use bare subject names — no -observability / -self-observability suffix.

• Renames test/e2e/grafana/compose/dashboards/cerberus-self.json to cerberus.json (git-tracked rename, history preserved).
• Mirrors the same rename for the k3d-stack copy at test/e2e/grafana/dashboards/cerberus-self.json.
• Updates the dashboard title: field from Cerberus - self-observability to plain Cerberus.
• Preserves the existing uid: "cerberus-self" so the /d/cerberus-self URL keeps working (Grafana provisioning keys off UID, not filename or title).

Reference updates

• docker-compose.yml — GF_DASHBOARDS_DEFAULT_HOME_DASHBOARD_PATH + service comment.
• test/e2e/grafana/compose/dashboards-provider.yaml — file mention in the provider comment.
• test/e2e/k3s/grafana-dashboards.yaml — file mention + ConfigMap name (grafana-dashboard-cerberus-self → grafana-dashboard-cerberus) + ConfigMap data key (cerberus-self.json → cerberus.json) + the embedded dashboard title.
• test/e2e/k3s/grafana.yaml — volume configMap.name reference matching the renamed ConfigMap.
• test/e2e/k3s/cerberus.yaml — comment.
• docs/observability.md — path reference.
• internal/api/admit/admit.go, internal/telemetry/metrics.go — prose comments.
• test/regression/k3s_env_test.go — assertion message + comment.
• test/e2e/playwright/{compose_grafana_smoke,iterate-all-dashboards,iterate-histogram-completeness,iterate-metrics-explorer,iterate-panel-kiosk,iterate-panel-shape,iterate-time-ranges}.spec.ts + helpers/{sweep.ts,README.md} — prose-only comments. The /d/cerberus-self URLs in compose_grafana_smoke.spec.ts are UID-based and were intentionally not touched.
• test/spec/promql/sum_by_rate_{range_step,dotted_source}.txtar + test/spec/logql/matcher_self_service.txtar — fixture header comments.

Test plan

• CI: compose-smoke (the dashboard is provisioned from the renamed file and served via GF_DASHBOARDS_DEFAULT_HOME_DASHBOARD_PATH).
• CI: dashboard smoke against k3d (renamed ConfigMap + data key consumed by grafana.yaml).
• CI: all required gates (check, lint, forbid-skip, compatibility, roundtrips).

🤖 Generated with Claude Code

[tsouza]

Folded into #701 (cerberus-self → cerberus rename rides with the other 3 dashboard renames there)