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).
