broker: resolve <parent> sentinel to topology parent at send time (#692)

hive-ag3nt/prompts/system.md

@@ -8,7 +8,7 @@ You are the hyperhive manager `{label}` (qualified: `{qualified_label}`) in a mu
 Tools (hyperhive surface):
 
 - `mcp__hyperhive__recv(wait_seconds?, max?)` — drain inbox messages (returns `(empty)` if nothing pending). Without `wait_seconds` (or with `0`) it returns immediately — a cheap "anything pending?" peek you can sprinkle between tool calls. To **wait** for work when you have nothing else useful to do this turn, call with a long wait (e.g. `wait_seconds: 180`, the max) — incoming messages wake you instantly, otherwise the call returns empty at the timeout. That's strictly better than a fixed `sleep` shell command: lower latency on new work, no busy-loop. `max` (default 1, cap 32) drains several queued messages in one call — the wake prompt tells you the pending count.
-- `mcp__hyperhive__send(to, body, in_reply_to?)` — message a peer (by their name) or the operator (recipient `operator`, surfaces in the dashboard). Use `to: "*"` to broadcast to all agents (they receive a hint that it's a broadcast and may not need action). Optional `in_reply_to: <message-id>` threads this message under a prior one — the dashboard and per-agent inbox render it with a `↳ reply` link. Some agents have a per-agent allow-list (`hyperhive.allowedRecipients` in their `agent.nix`) — if so the tool refuses recipients outside the list with a clear error; route through the manager (`send(to: "manager", …)`) which is always reachable.
+- `mcp__hyperhive__send(to, body, in_reply_to?)` — message a peer (by their name) or the operator (recipient `operator`, surfaces in the dashboard). Use `to: "*"` to broadcast to all agents (they receive a hint that it's a broadcast and may not need action). Use `to: "<parent>"` to address your structural parent without hardcoding their name — hive-c0re rewrites it at delivery time per `topology.json`, falling back to `operator` if you're a root agent. Lets the operator reparent you at runtime with zero change on your side. Optional `in_reply_to: <message-id>` threads this message under a prior one — the dashboard and per-agent inbox render it with a `↳ reply` link. Some agents have a per-agent allow-list (`hyperhive.allowedRecipients` in their `agent.nix`) — if so the tool refuses recipients outside the list with a clear error; route through the manager (`send(to: "manager", …)`) which is always reachable.
 <!-- role:agent -->
 - (some agents only) **extra MCP tools** surfaced as `mcp__<server>__<tool>` — these are agent-specific (matrix client, scraper, db connector, etc.) declared in your `agent.nix` under `hyperhive.extraMcpServers`. Treat them as first-class tools alongside the hyperhive surface; the operator already auto-approved them at deploy time.
 <!-- /role:agent -->

hive-ag3nt/src/mcp.rs

@@ -1883,6 +1883,16 @@ fn check_send_allowed(to: &str) -> Result<(), String> {
         // to ask for help.
         return Ok(());
     }
+    if to == hive_sh4re::PARENT_RECIPIENT {
+        // Always allow `<parent>` — same escape-hatch rationale as
+        // the manager exception. The allow-list constrains peer
+        // chatter, not the structural reporting line; the operator
+        // can rewire who the parent IS via `set_parent` without
+        // having to remember to update the per-agent allow-list.
+        // The broker resolves the sentinel to the real parent label
+        // on the host side per topology.json (#692).
+        return Ok(());
+    }
     let Ok(raw) = std::fs::read_to_string(SEND_ALLOW_PATH) else {
         return Ok(()); // file missing → no policy configured → unrestricted
     };