docs(#2627): add READMEs for hive-jobq + the socket wire crates

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).

hive-jobq/Cargo.toml

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

hive-jobq/README.md

@@ -0,0 +1,58 @@
+# hive-jobq
+
+A persistent job-DAG scheduler, extracted from hive-c0re's in-tree `job_queue`
+as a **domain-agnostic** library. It schedules a single persistent graph of
+nodes over named resources; it knows nothing about containers, rebuilds, or any
+hyperhive type — the node payload `N` and resource name `R` are both generic, so
+the caller supplies its own domain.
+
+## When to use it
+
+Reach for this crate whenever you need to run a DAG of interdependent work items
+under bounded, named concurrency — the hive-c0re rebuild/lifecycle queue is the
+first consumer, but nothing here is specific to it. The caller defines the node
+kinds, wires deps, and supplies a runner; the scheduler decides what can start.
+
+## Model
+
+One **persistent graph** for the whole system, not a DAG per job. Enqueuing
+inserts a self-contained sub-DAG and returns the new node ids; the scheduler
+runs a continuous loop, starting every node whose deps are satisfied:
+
+- **Resource deps** are named counting semaphores over a caller-chosen type `R`
+  — e.g. `build-slot` (capacity N), `agent/<name>` (capacity 1), or any
+  unconfigured name (capacity 1, created on use). A node acquires *all* its
+  resource deps atomically at start (all-or-nothing) — no hold-and-wait, so no
+  deadlock.
+- **Node deps** wait on another node per `DepWhen`: `AfterOk` needs success (a
+  failed dep cancels the dependent), `AfterAny` only needs terminal.
+
+A node carries two independent axes: its `Dep`s (ordering + resource needs) and
+its `parent` (structural grouping). The **parent chain**, not the node edges, is
+what the scheduler consults for resource re-entrancy: a resource unit is held
+for the acquiring node *plus its whole parent subtree*, and a descendant needing
+a resource an ancestor already holds re-uses that grant (a re-entrant borrow,
+one branch at a time) rather than taking a fresh unit.
+
+A `NodeId` is opaque, stable, and monotonic (safe to persist). The scheduler is
+single-threaded — it owns the resource table and mutates it directly.
+
+## Shape
+
+- **`Graph<N, R>`** — the persistent node store. `insert` mints ids and
+  validates dep/parent references; `set_state` is the single state-transition
+  choke point (and where each node's lifecycle timestamps —
+  `started_at` / `finished_at`, `DateTime<Utc>` — are stamped).
+- **`Node<N, R>`** — `{ id, parent, payload, deps, state, started_at,
+  finished_at, error }`. All fields public; derives serde for persistence + the
+  wire.
+- **`Scheduler<N, R>`** — drives the graph: `settle()` starts every ready node
+  (acquiring resources atomically), `complete(id, outcome)` reports a finished
+  node's result and rolls terminality up the parent chain, releasing grants once
+  a subtree is done. `Outcome::{Done, Failed(String)}` — the failure reason
+  rides `Failed` onto the node's `error`.
+- **`ResourceTable<R>`** — per-name capacities; unconfigured names default to
+  capacity 1.
+
+See the crate-root and `scheduler` module `//!` docs for the full borrow/release
+model.