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-core-agent-sock/Cargo.toml

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

hive-core-agent-sock/README.md

@@ -0,0 +1,21 @@
+# hive-core-agent-sock
+
+Wire types for the **per-agent + manager socket** (`/run/hive/mcp.sock`) — the
+unified `Request` / `Response` protocol spoken between an agent's in-container
+harness (and the manager) and the `hive-c0re` daemon.
+
+## Why it's its own crate
+
+Re-homed out of `hive-sh4re` so this one socket owns its protocol crate,
+mirroring the `hive-host-sock` / `hive-priv-sock` splits. The shared payload
+types it references (`Message`, `LooseEnd`, `Approval`, …) stay in `hive-sh4re`,
+which this crate depends on — this crate is just the request/response envelope
+for this socket.
+
+## Not to be confused with `hive-agent-sock`
+
+This is the **host-served** protocol: the harness talks *out* to `hive-c0re`
+over `/run/hive/mcp.sock` (broker sends, approvals, questions, lifecycle).
+`hive-agent-sock` is the separate *in-container* socket the harness serves to
+its own local producers — that one never leaves the container. See
+`docs/boundary.md` for the socket topology.