docs(#2627): add READMEs for the remaining infra crates

Second increment of the per-crate README effort, covering the rest of the
infra/wire/priv column: hive-priv, hive-metric, hive-types, hive-sh4re,
hive-core-agent-sock, hive-agent-sock. Same shape as the first batch —
purpose + when-to-use, and point at the crate-root //! docs plus the
relevant docs/ pages rather than duplicating them. Wires
readme = "README.md" into each Cargo.toml [package].

Disjoint from the batch-1 crates, so the two increments compose cleanly.

hive-metric/Cargo.toml

@@ -1,6 +1,7 @@
 [package]
 name = "hive-metric"
 version.workspace = true
+readme = "README.md"
 edition.workspace = true
 
 [[bin]]

hive-metric/README.md

@@ -0,0 +1,26 @@
+# hive-metric
+
+A tiny CLI that pushes **a single labeled metric** to the OTEL collector, then
+exits. Built on the OpenTelemetry Rust SDK with the OTLP HTTP/JSON exporter.
+
+## When to use it
+
+For emitting a one-off counter or gauge from a shell script or systemd unit
+without embedding an OTEL SDK in the caller — the harness and various hive
+scripts use it to record per-agent metrics.
+
+## Usage
+
+```text
+hive-metric <name> <value> [--type counter|gauge] [--labels key=value ...]
+```
+
+Standard OTEL environment variables are read automatically by the SDK:
+
+- `OTEL_EXPORTER_OTLP_ENDPOINT` — collector URL (**required**)
+- `OTEL_EXPORTER_OTLP_HEADERS` — auth headers (`Key=Value,...`)
+- `OTEL_RESOURCE_ATTRIBUTES` — resource labels (`k=v,...`)
+
+The harness populates all of these per-agent when
+`services.hyperhive.otel.enable = true`. See `docs/observability.md` for the
+collector setup and the metrics hyperhive exports.