docs(observability): refresh docs for service-registry end state

After the observability-service-registry slices removed the observability
env vars and the file-SD writer, several docs still instructed readers to
set vars that no longer exist. Updated the live config instructions;
historical decision-log entries are left intact.

- README.md: removed VITE_GRAFANA_URL/VITE_PROMETHEUS_URL/ALERTMANAGER_URL
  from compose examples and the env-var block; added a note that
  observability is configured on the Services page; updated Notes.
- frontend/README.md: dropped the stale VITE_* deep-link sentence.
- docs/REQUIREMENTS.md: fixed one stale trailing phrase in the
  externalization decision-log entry (VITE_* no longer "remain").
- docs/monitoring-logging-design.md: added a "Superseded mechanisms" note
  under Implementation Plan so the Phase 2/3 file-SD + alertmanager_url
  details read as historical, not current wiring.
- context.md: strengthened the status banner to cover the env->service-
  registry and file-SD->http_sd_configs shift; body marked historical.

.env.example is assistant-edit-blocked; updated replacement text provided
to the user separately.

docs/monitoring-logging-design.md

@@ -307,6 +307,17 @@ The existing POSIX remote collector will be removed, and the Python backup alert
 
 ## Implementation Plan
 
+> **Superseded mechanisms (2026-06-23):** Phases 2 and 3 originally wired Manage
+> to a co-deployed Prometheus/Alertmanager via a shared-file **file-SD bridge**
+> (`PROMETHEUS_FILE_SD_DIR` + `write_prometheus_targets`) and an
+> `alertmanager_url` env var. Both have been removed. Manage now resolves
+> Alertmanager/Grafana/Prometheus from the **service registry**, and external
+> Prometheus instances consume node-exporter targets via **`http_sd_configs`**
+> against `GET /api/monitoring/prometheus-targets`. Treat the Phase 2/3
+> file/config details below as the historical implementation, not the current
+> wiring. See `openspec/changes/observability-service-registry/` and the
+> decision log in `docs/REQUIREMENTS.md`.
+
 ### Phase 0 — Foundation and Cleanup
 
 - [x] Add `prometheus-client` and `python-json-logger` to `backend/pyproject.toml`.