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-types/Cargo.toml

@@ -2,6 +2,7 @@
 name = "hive-types"
 edition.workspace = true
 version.workspace = true
+readme = "README.md"
 
 [lints]
 workspace = true

hive-types/README.md

@@ -0,0 +1,18 @@
+# hive-types
+
+Foundational shared **newtypes** for the hyperhive workspace. A zero-dependency
+(bar `serde`) leaf crate, so every wire-type crate (`hive-sh4re`,
+`hive-host-sock`, `hive-core-agent-sock`) and both binaries (`hive-c0re`,
+`hivectl`) can share them without cross-crate coupling and without growing
+`hive-sh4re`.
+
+## What's here
+
+- **`Ident`** — a validated agent-name newtype. Typing agent-name fields as
+  `Ident` (rather than bare `String`) gets serde-validated parsing **at the
+  socket boundary for free**: a malformed name is rejected at deserialize time
+  instead of flowing into a handler that has to re-validate by hand.
+
+The crate is deliberately a leaf with the smallest possible dependency footprint
+so anything in the workspace can depend on it. See `docs/conventions.md`
+("identity = socket") for how agent identity is modelled across the sockets.