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-agent-sock/Cargo.toml

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

hive-agent-sock/README.md

@@ -0,0 +1,26 @@
+# hive-agent-sock
+
+Wire types for the **in-agent socket** — the one the `hive-agent` harness serves
+*inside* the container to its local producers (the matrix / bash MCP daemons and
+`forge_notify`). Unlike the host-served sockets, this one **never leaves the
+container**.
+
+## What it carries
+
+- the loose-ends-v2 **todo** op family
+- the harness-local **reminder** op family
+
+More in-agent request families may be added over time — the socket is
+deliberately named for the agent, not for the todos.
+
+## Why in-container
+
+The harness owns the todo + reminder stores locally and signals its own turn
+loop directly, so `hive-c0re` is not in either path: no broker round-trip, no
+long-poll, no marker files. That locality is the point — it's also what lets
+these stores travel with the agent for hive portability.
+
+## Not to be confused with `hive-core-agent-sock`
+
+That crate is the *host*-served core↔agent protocol on `/run/hive/mcp.sock`
+(the harness talking out to `hive-c0re`). This socket is purely intra-container.