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/src/lib.rs

@@ -44,7 +44,7 @@ pub mod workers;
 // Root re-exports: keep every pre-grouping `crate::<module>` /
 // `hive_c0re::<module>` path compiling without touching consumers.
 pub use agent_config::{capabilities, limits, tool_groups, topology};
-pub use stats::{container_stats, hive_stats, host_stats, sweep_health, warnings};
+pub use stats::{container_stats, hive_stats, host_stats, otel_metrics, sweep_health, warnings};
 pub use stores::{
     approvals, audit_log, broker, build_logs, db, operator_questions, power, scheduled_prompts,
 };