docs(#2627): add READMEs for hive-jobq + the socket wire crates

Adds crate READMEs (matching the hive-claude precedent) and wires
readme = "README.md" into each Cargo.toml [package] for hive-jobq,
hive-host-sock, and hive-priv-sock — the crates squarely in the infra
lane. Each README leads with purpose + when-to-use and points at the
crate-root //! docs for depth rather than duplicating them.

First increment of the per-crate-README effort; the shape here is the
proposed template for the remaining crates (see issue discussion).

hive-host-sock/Cargo.toml

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

hive-host-sock/README.md

@@ -0,0 +1,24 @@
+# hive-host-sock
+
+Wire types for the **host admin socket** (`/run/hyperhive/host.sock`) — the
+host-control protocol spoken between the `hivectl` operator CLI and the
+`hive-c0re` daemon.
+
+## Why it's its own crate
+
+Re-homed out of `hive-sh4re` so a standalone `hivectl` depends on **just this
+protocol crate** instead of the whole daemon-shared crate. `hivectl` drives the
+full hive (spawn / kill / destroy / rebuild / deploy) over this socket without
+linking `hive-c0re`; keeping the request/response shapes here is what makes that
+thin dependency possible.
+
+## Shape
+
+Serde-derived request/response enums for the host admin protocol. The larger
+shared payload types some variants reference (`Approval`, `AgentStatusRow`,
+`jobs::DagView`) stay in `hive-sh4re` — this crate is only the protocol
+envelope, no server or client implementation.
+
+See `docs/boundary.md` (host admin socket access) for the trust model around who
+may connect to the socket, and `hive-priv-sock` for the sibling split on the
+privileged-helper socket.