Land argus's review nit from #2654 (it was pushed to that PR's branch as
fc4230f2 but got orphaned when the branch was merged + auto-deleted before
the fix landed). The hive-sh4re README opening said 'Agent / Manager
request + response shapes', which reads as if the wire envelopes still live
here; they moved to the per-socket crates. Now says 'shared payload
vocabulary' and notes where the envelopes live.
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.
