feat(#2007): export per-agent container cpu/mem/disk via otel

hive-c0re already samples each agent container's cgroup load for the
dashboard (stats/container_stats.rs); this rides those gauges out to the
configured OTLP endpoint, reusing the existing services.hyperhive.otel
config (endpoint + auth header) — no new toggle.

- New stats/otel_metrics.rs: exports via the OpenTelemetry Rust SDK (same
  crates as hive-metric) with the semconv container.* metric names +
  container.name attribute so off-the-shelf OTel/Grafana dashboards work,
  plus the hive agent label. container.cpu.time (counter, s, from cumulative
  cpu.stat usage_usec), container.memory.usage, container.memory.usage.limit;
  memory peak / on-disk storage / instantaneous cpu percent stay hyperhive.*
  custom (no semconv equivalent). Observable instruments read a shared
  snapshot an async task refreshes (gather() is async; SDK callbacks sync).
- container_stats: expose cpu_time_usec (cumulative) on ContainerResource.
- The OTLP auth header is loaded onto hive-c0re's own unit via systemd
  LoadCredential and read from $CREDENTIALS_DIRECTORY/otel-headers.
- docs/observability.md documents the host-emitted semconv metrics.

Host-side export, so it covers containers even when their agent is idle.

hive-c0re/Cargo.toml

@@ -17,6 +17,18 @@ url.workspace = true
 clap.workspace = true
 clap_complete.workspace = true
 clap-markdown = "0.1"
+# OTEL SDK for the per-agent container-resource metrics exporter
+# (stats/otel_metrics.rs). Same versions/features as hive-metric — the
+# blocking OTLP client is deliberate: the metrics SDK's PeriodicReader runs
+# on a background thread with no Tokio reactor, where the async client panics.
+opentelemetry = "0.32"
+opentelemetry_sdk = { version = "0.32", features = ["metrics"] }
+opentelemetry-otlp = { version = "0.32", default-features = false, features = [
+  "metrics",
+  "http-json",
+  "reqwest-blocking-client",
+  "reqwest-rustls",
+] }
 indicatif.workspace = true
 hive-sh4re.workspace = true
 hive-host-sock.workspace = true