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- docs/monitoring-logging-design.md: added a "Superseded mechanisms" note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.

context.md

@@ -1,13 +1,19 @@
 # Code Context
 
-> **Status (2026-06-23):** Manage no longer deploys an observability stack.
-> The root `docker-compose.yml` / `docker-compose.dev.yml` ship **only** the
-> backend and frontend; Grafana, Prometheus, Loki, Alertmanager, Alloy, and Node
-> Exporter were removed from them. Manage connects to **existing** instances.
-> The standalone example stack lives in `docker-compose.observability.yml`. Some
-> snippets below still reference the former in-compose services and are kept as
-> historical context; treat `docker-compose.observability.yml` as authoritative
-> for the stack layout.
+> **Status (2026-06-23):** Manage no longer deploys an observability stack,
+> and observability is no longer env-configured. The root `docker-compose.yml` /
+> `docker-compose.dev.yml` ship **only** the backend and frontend; Grafana,
+> Prometheus, Loki, Alertmanager, Alloy, and Node Exporter were removed from
+> them. Manage connects to **existing** instances, and Alertmanager/Grafana/
+> Prometheus are now **service-registry instances** configured in the UI (no
+> `ALERTMANAGER_URL`, `VITE_GRAFANA_URL`, `VITE_PROMETHEUS_URL`, or
+> `PROMETHEUS_FILE_SD_DIR` env vars). The file-SD writer was removed; external
+> Prometheus uses `http_sd_configs` against `GET /api/monitoring/prometheus-targets`.
+> The standalone example stack lives in `docker-compose.observability.yml`. The
+> detailed snippets below describe the **former** in-compose, env-configured
+> model and are kept as historical context only; treat the service registry
+> (Services page UI + `openspec/changes/observability-service-registry/`) and
+> `docker-compose.observability.yml` as authoritative.
 
 ## Files Retrieved