hive-c0re: push HelperEvent::Spawned as a todo, not a broker message

Follow-up from #2955 (mara: 'make core able to give agent a todo').
First migration slice: Spawned was pure FYI-check-when-convenient
material, not something needing an immediate turn.

Coordinator::push_todo/push_todo_submitter do a best-effort live dial
of the target agent's own hive-agent-sock (hive_host_sock::agent_todo_
socket), sending the exact UpsertTodo request in-container producers
(matrix/bash/forge-notify) already send. Push, not queue: agent
offline (socket absent) or dial failure is a silent no-op, no retry,
no fallback delivery -- matches mara's 'not available if offline'
call exactly.

HelperEvent::Spawned removed entirely (enum variant + all 3 call
sites migrated: handle_spawn's two arms, finish_approval's Spawn
approval-kind arm) rather than kept alongside a translation layer --
per mara's correction on the first design attempt, migrating the
producer means deleting the old path, not bridging it.

Verified: cargo build/clippy/test -p hive-c0re -p hive-host-sock
-p hive-sh4re clean (318 tests), nix fmt clean.

hive-c0re/src/server.rs

@@ -297,22 +297,30 @@ async fn handle_spawn(coord: &Arc<Coordinator>, name: &str) -> Result<HostRespon
             // Bind the MCP listener now that the container is starting up.
             // The harness connects to this socket on its first turn.
             coord.register_agent(name)?;
-            coord.notify_manager(&hive_sh4re::HelperEvent::Spawned {
-                agent: name.to_owned(),
-                ok: true,
-                note: None,
-            });
+            coord
+                .push_todo(
+                    hive_sh4re::MANAGER_AGENT,
+                    "core",
+                    Some(format!("spawned:{name}")),
+                    format!("agent '{name}' spawned"),
+                    None,
+                )
+                .await;
             // Update tmpfiles.d so the new agent's dirs survive a reboot.
             tokio::spawn(lifecycle::sync_tmpfiles());
         }
         Err(e) => {
             // Spawn failed: register_agent was never called, so there is
             // nothing to unregister. Notify the manager and propagate.
-            coord.notify_manager(&hive_sh4re::HelperEvent::Spawned {
-                agent: name.to_owned(),
-                ok: false,
-                note: Some(format!("{e:#}")),
-            });
+            coord
+                .push_todo(
+                    hive_sh4re::MANAGER_AGENT,
+                    "core",
+                    Some(format!("spawned:{name}")),
+                    format!("agent '{name}' spawn FAILED: {e:#}"),
+                    None,
+                )
+                .await;
             return Err(e);
         }
     }