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-priv/Cargo.toml

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

hive-priv/README.md

@@ -0,0 +1,32 @@
+# hive-priv
+
+The minimal **root privileged-helper** for hive-c0re. It runs as root and
+exposes a narrow unix socket at `/run/hive/priv.sock` that accepts `PrivRequest`
+JSON lines and performs only the handful of operations that genuinely require
+root — bind-mount edits, `nsenter` into a container, btrfs subvolume ops. All
+coordination logic (broker, HTTP, scheduling) stays in the *unprivileged*
+`hive-c0re` process, which delegates here.
+
+## Why it exists
+
+Privsep. `hive-c0re` runs as the unprivileged `hive-core` user so a bug or a
+prompt-injection in the large daemon can't directly wield root. The few root
+operations it needs are funnelled through this small, auditable helper instead.
+See `docs/boundary.md` and `docs/security.md` for the privilege boundary.
+
+## Security model
+
+- **Strict allowlist.** Every request is validated against a container-name
+  allowlist before any filesystem or process operation — only names matching the
+  hive convention (`h-*`, the manager container, known sibling service
+  containers) are accepted.
+- **No pass-through.** Every `PrivRequest` variant maps to a single known
+  operation; there is no arbitrary-command escape hatch.
+- **Socket-activated, always.** systemd binds `/run/hive/priv.sock`
+  (`SocketGroup=hive-core`, `0660`) and passes the listener as fd 3
+  (`LISTEN_FDS`); the helper requires this and has no self-bind fallback, so dev
+  and prod take the identical path and the group grant always holds.
+
+The wire contract (`PrivRequest` / response types) lives in the separate
+`hive-priv-sock` crate so this root binary depends on just the protocol shapes,
+not the whole daemon-shared crate.