docs: move privsep socket-activation + child-state rw rationale out of code comments

docs/boundary.md

@@ -59,3 +59,19 @@ The `area:ops` issues followed this sequencing:
    runs as the unprivileged `hive-core` user and delegates root
    operations to `hive-priv`, a narrow socket-activated helper. See
    [`docs/security.md`](security.md) for the privilege boundary table.
+
+### hive-priv socket activation
+
+`hive-priv` is **always** socket-activated by the `hive-priv.socket`
+systemd unit. The unit binds `/run/hive/priv.sock` with
+`SocketGroup=hive-core` and mode `0660` and passes the ready listener
+to the helper as fd 3 (`LISTEN_FDS`). The helper requires this and
+bails if it isn't socket-activated — there is intentionally no
+self-bind fallback.
+
+Dropping the old fallback removed a dev/prod divergence: when
+`hive-priv` bound the socket itself it created the file owned by
+root's primary group rather than `hive-core`, so a `hive-core` client
+couldn't connect the way the socket unit's `SocketGroup` grant
+intends. Requiring socket activation everywhere means dev and prod
+take the exact same path and the group grant always holds.

docs/persistence.md

@@ -220,6 +220,20 @@ Under `/var/lib/hyperhive/agents/<name>/`:
   - `hyperhive-turn-stats.sqlite` — per-turn timing stats.
   - `hyperhive-model` — single-line model name override file.
 
+### Parent access to child state
+
+A parent agent gets each direct child's `state`, `harness`, and
+`config` dirs bind-mounted **read-write** (`bind_child_agent_dirs` in
+`lifecycle.rs`). The RW on `state` is deliberate, not an oversight: a
+parent manages its children, which includes writing into a child's
+state for recovery (e.g. seeding notes, clearing a stuck sentinel) as
+well as reading it. `config` is RW because the parent authors proposed
+config changes for the child (the approval flow commits into the
+child's config repo), and `harness` is RW for the same management
+reasons. Per-child isolation still holds: a container only ever has
+its *own* dirs plus its direct children's bind-mounted, never a
+sibling's.
+
 Under `/var/lib/hyperhive/applied/<name>/` — the hive-c0re-only
 applied repo. Tracks `flake.nix` (module-only boilerplate; never
 edited after first spawn) + `agent.nix` (the actual config; the