docs(observability): document OTEL configuration options

Add docs/observability.md covering all services.hyperhive.otel.*
options: enable, endpoint, protocol, headersCredential,
extraResourceAttributes, debug (new in cb0a66147a), and
metricIntervalMs.

Includes:
- Built-in OTEL_RESOURCE_ATTRIBUTES labels (service.name, agent, hive, swarm)
- Cumulative temporality note (avoids Prometheus DELTA drop)
- Network note for host-side collectors on non-standard ports,
  cross-referencing docs/network.md exposeHostPorts

Also:
- CLAUDE.md: add reading-path entry for the new doc
- docs/network.md: link the OTEL mention to observability.md

Closes no issue — gap found during doc sweep.

CLAUDE.md

@@ -113,6 +113,9 @@ read them à la carte.
 - **"What is `/knowledge`? How does the hive-wide knowledge repo sync,
   and how do I contribute a document?"** →
   [`docs/knowledge.md`](docs/knowledge.md).
+- **"How do I export Claude Code metrics (tokens, cost, tool calls) to
+  a Prometheus/Grafana collector? What OTEL options are available?"** →
+  [`docs/observability.md`](docs/observability.md).
 
 ## Conventions & process