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).
Split the priv-socket wire types (PrivRequest/PrivResponse/PrivEvent and
friends) out of hive-sh4re into their own hive-priv-sock crate, mirroring
the existing hive-host-sock split. hive-priv — the root-privileged
helper — now depends on just this narrow protocol crate instead of the
much larger daemon-shared crate, shrinking its dependency surface and
making the privsep boundary easier to audit. No server/client
implementation lives here, only the wire contract; hive-c0re still
depends on hive-sh4re directly for everything else.
