topology: drop the parent field and the hierarchy it fed

`topology.json` was a map of `name -> parent | null`, and that value fed
the whole agent hierarchy: `<parent>` / `<children>` recipient sentinels,
the reparenting API (CLI verb, wire verb, dashboard endpoints, DAG node),
the dashboard tree, the rebuild depth sort, and an unconditional
bind-mount grant giving every agent RW on its direct children's state.

Per the operator's ruling the field goes, and with it all of the above.
The file survives as what remains once the value is gone: the roster of
agent names, which is the set `ManageRootAgent` grants mounts over. It is
now a JSON array; `read` still accepts the old map shape and keeps its
keys, so a hive that upgrades across this does not blank its roster (and
so no capability holder loses its mounts for the length of that window).

Two sites kept their behaviour under a different recipient rather than
losing it. Both addressed `<parent>`, which the broker already resolved to
`operator` for a root agent, and every agent is now what that fallback
called a root:

- the harness's turn-failure / plugin-failure notification
  (`Surface::send_to_parent` -> `send_to_operator`), and
- the send allow-list's always-permitted escape hatch, so an agent with a
  restrictive allow-list still has a way to say it is stuck.

What is NOT preserved, deliberately: an agent with no capability no longer
sees any other agent's dirs. `ManageRootAgent`'s own grant is unchanged --
still every agent in the roster, still state RW + config RO, still no
`harness`.

The dashboard's reparenting control (the M0V3 picker) is deleted with its
CSS. The tree rendering that reads `ContainerView.parent` is left for the
frontend owner -- it degrades to a flat list with the field gone.

hivectl/src/agents.rs

@@ -1,6 +1,6 @@
 //! `hivectl agent <name>` — everything scoped to ONE managed agent:
 //! container lifecycle over the host admin socket (restart/pause/resume/
-//! spawn/kill/destroy/rebuild/set-parent/set-limits/choom/watch), plus the
+//! spawn/kill/destroy/rebuild/set-limits/choom/watch), plus the
 //! `quota` and `subvol` groups, whose handlers live in their own modules.
 //! `agents_list` (`hivectl list-agents`) is the one genuinely hive-wide
 //! read that lives in this module too since it shares the same daemon
@@ -172,15 +172,14 @@ pub(crate) async fn agents_list(socket: &Path, json: bool) -> Result<()> {
         }
         s
     };
-    let headers = ["NAME", "STATUS", "REV", "PARENT", "REMIND"];
-    let table: Vec<[String; 5]> = rows
+    let headers = ["NAME", "STATUS", "REV", "REMIND"];
+    let table: Vec<[String; 4]> = rows
         .iter()
         .map(|r| {
             [
                 r.name.clone(),
                 status_of(r),
                 r.deployed_sha.clone().unwrap_or_else(|| "-".to_owned()),
-                r.parent.clone().unwrap_or_else(|| "-".to_owned()),
                 if r.pending_reminders > 0 {
                     r.pending_reminders.to_string()
                 } else {
@@ -291,18 +290,6 @@ pub(crate) async fn run_agent(socket: &Path, name: &str, cmd: AgentCmd) -> Resul
             let name = crate::util::parse_ident(name)?;
             render(crate::client::request(socket, HostRequest::Rebuild { name }).await?)
         }
-        AgentCmd::SetParent { parent, root } => {
-            let child = crate::util::parse_ident(name)?;
-            let new_parent = if root {
-                None
-            } else {
-                parent.map(|p| crate::util::parse_ident(&p)).transpose()?
-            };
-            render(
-                crate::client::request(socket, HostRequest::SetParent { child, new_parent })
-                    .await?,
-            )
-        }
         AgentCmd::SetLimits {
             cpu_quota,
             memory_max,

hivectl/src/cli.rs

@@ -507,16 +507,6 @@ pub enum AgentCmd {
     },
     /// Apply pending config to this managed container.
     Rebuild,
-    /// Move this agent in the topology tree — under a new parent, or to
-    /// root.
-    SetParent {
-        /// New parent agent name. Mutually exclusive with `--root`.
-        #[arg(long, conflicts_with = "root", required_unless_present = "root")]
-        parent: Option<String>,
-        /// Promote this agent to root (no parent).
-        #[arg(long)]
-        root: bool,
-    },
     /// Declare this agent's CPU/memory limits, overriding the hive-wide
     /// defaults.
     ///