topology: require --parent/--root explicitly + fix stale form doc (#492 nits)

hive-c0re/src/dashboard.rs

@@ -852,9 +852,13 @@ struct RequestSpawnForm {
 
 /// `POST /api/topology/set-parent` body. `child` is required.
 /// `new_parent` may be:
-/// - omitted entirely (form field absent) → no-op error,
-/// - empty string → promote to root,
+/// - absent or empty / whitespace-only → promote to root,
 /// - non-empty → new parent's logical name.
+///
+/// (The CLI surface gates "no parent specified" behind an explicit
+/// `--root` flag for safety; the HTTP surface is permissive
+/// because the dashboard form encodes "no value" as the empty
+/// string for the optional radio-group input.)
 #[derive(Deserialize)]
 struct SetParentForm {
     child: String,

hive-c0re/src/main.rs

@@ -109,7 +109,11 @@ enum Cmd {
     SetParent {
         child: String,
         /// New parent agent name. Mutually exclusive with `--root`.
-        #[arg(long, conflicts_with = "root")]
+        /// Exactly one of `--parent` / `--root` is required — clap
+        /// rejects both-absent calls so a fat-fingered
+        /// `hive-c0re set-parent alice` doesn't silently promote
+        /// alice to root (argus flag on PR #492).
+        #[arg(long, conflicts_with = "root", required_unless_present = "root")]
         parent: Option<String>,
         /// Promote `child` to root (no parent).
         #[arg(long)]