fix(3044): a parent's mount of a child's config is read-only

The parent's copy is for reading a child's config; a change to it is a PR
on the child's repo, made from a clone and merged after review. A
writable mount is a second path to the same file that skips that review,
which makes the boundary a convention rather than a permission.

Confirmed with ruth before flipping: it clones from the forge and opens a
PR, including for a brand-new child's first config.

The prose was the larger half. docs/approvals.md did not merely describe
the old mount, it *instructed* agents to use it ("can therefore edit,
commit, and submit changes for any of its direct children directly inside
its container"), and the doc comment in host_config.rs asserted a
dependency that never existed: the InitConfig seed runs as hive-c0re
against the host path, and read_only on a bind constrains writers inside
the container only. That comment is what produced issue #3206, now closed
as invalid.

hive-c0re/src/lifecycle/host_config.rs

@@ -71,37 +71,46 @@ async fn systemd_daemon_reload() -> Result<()> {
 /// inside the container.
 pub const CONTAINER_MANAGER_APPLIED_MOUNT: &str = "/applied";
 
-/// Append bind flags for `child`'s state and config dirs into `binds`,
-/// read-write. See docs/persistence.md ("Parent access to child state")
-/// for what a parent may touch and why. Creates missing host-side
-/// directories so nspawn doesn't refuse to start; missing dirs are
-/// non-fatal.
+/// Append bind flags for `child`'s state and config dirs into `binds`.
+/// See docs/persistence.md ("Parent access to child state") for what a
+/// parent may touch and why. Creates missing host-side directories so
+/// nspawn doesn't refuse to start; missing dirs are non-fatal.
 ///
-/// **`harness` is deliberately absent.** It holds the child's own runtime
-/// material — `bash-tasks/`, turn-stats and event sqlite dbs — none of
-/// which a parent has a stated reason to read, let alone write. It was
-/// mounted only because the loop treated all three dirs alike; the three
-/// have three different answers, and the uniformity is what hid that.
-/// hive-c0re still reads a child's harness dir directly on the host
-/// (`stats::hive_stats`), which needs no bind mount into the parent.
+/// **Three dirs, three different answers** — the uniformity of the
+/// original loop is what hid that:
 ///
-/// `config` is RW here **pending** the sequenced change: the ruling is
-/// that it becomes read-only, but `request_init_config` still has the
-/// manager seed a new child's config in place, so flipping the mount
-/// before relocating that step breaks agent creation hive-wide.
+/// - `state` is **read-write**: a parent reads and writes a child's notes
+///   to recover it, which is the one case that needs to work while the
+///   child is down.
+/// - `config` is **read-only**, and read-only for the *parent* is the
+///   point: a config change is a PR against the child's repo on the
+///   forge, reviewed and merged, never an edit in place. A writable mount
+///   here is a second path to the same file that skips the review — the
+///   boundary would then be a convention rather than a permission.
+/// - `harness` is **absent entirely**. It holds the child's own runtime
+///   material — `bash-tasks/`, turn-stats and event sqlite dbs — none of
+///   which a parent has a stated reason to read, let alone write.
+///   hive-c0re still reads it directly on the host (`stats::hive_stats`),
+///   which needs no bind mount into the parent.
+///
+/// ⚠️ The seeding done at `InitConfig` approval is **not** affected by the
+/// `config` flag and must not be read as a reason to widen it: that runs
+/// as hive-c0re against the host path (see `actions.rs`, which seeds the
+/// repo and wires its forge remote inline), and `read_only` on a bind
+/// constrains writers *inside* the container only.
 fn bind_child_agent_dirs(child: &str, binds: &mut Vec<BindMount>) {
     let Ok(child) = hive_types::Ident::parse(child) else {
         tracing::warn!(%child, "skipping child bind: invalid agent name");
         return;
     };
     let child_root = crate::paths::agent_state_dir(&child);
-    for sub in ["state", "config"] {
+    for (sub, read_only) in [("state", false), ("config", true)] {
         let host = child_root.join(sub);
         let _ = std::fs::create_dir_all(&host);
         binds.push(BindMount {
             host_path: host.to_string_lossy().into_owned(),
             container_path: format!("/agents/{child}/{sub}"),
-            read_only: false,
+            read_only,
         });
     }
 }
@@ -252,9 +261,8 @@ async fn set_nspawn_flags(
     });
 
     // Topology-driven child mounts: every direct child of this agent gets
-    // its state, harness, and config dirs bind-mounted RW (parent reads +
-    // writes child state for recovery, and manages config). See
-    // `bind_child_agent_dirs`.
+    // its state dir RW and its config dir RO. See `bind_child_agent_dirs`
+    // for why each is what it is.
     let direct_children = crate::topology::children_of(agent_name);
     for child in &direct_children {
         bind_child_agent_dirs(child, &mut binds);