docs: fix stale wake caveat, add <parent> sentinel + systemd path gotcha

turn-loop.md:
- remove stale "Agent-only today" note from hive wake (fixed by #698)
- add <parent> sentinel to send tool description (added by #703)

gotchas.md:
- add systemd.services.*.path /bin auto-append gotcha (hit by #672/#705)

docs/gotchas.md

@@ -39,6 +39,28 @@ claude's Bash tool without editing the service definition.
 `environment.HYPERHIVE_GIT` bakes git's absolute path in (read by
 `lifecycle::git_command()`) for the host.
 
+## `systemd.services.*.path` appends `/bin` to every entry
+
+NixOS's `systemd.services.<unit>.path` list feeds every entry through
+`lib.makeBinPath`, which **appends `/bin` unconditionally**. That's
+the right thing for Nix packages (their `outPath` is the store root,
+not the `bin/` subdir), but it bites when you pass a string that
+already ends with `/bin`:
+
+```nix
+# ❌ /run/wrappers/bin → /run/wrappers/bin/bin (does not exist)
+path = [ "/run/wrappers/bin" "/run/current-system/sw" ];
+
+# ✅ /run/wrappers → /run/wrappers/bin  (the real wrappers dir)
+path = [ "/run/wrappers" "/run/current-system/sw" ];
+```
+
+The bug is silent: `nix eval` succeeds, the unit starts, but PATH
+contains a non-existent directory. The first symptom is usually
+`sudo: must be owned by uid 0 and have the setuid bit set` because
+the setuid sudo wrapper lives at `/run/wrappers/bin/sudo` and
+the path entry resolves to `/run/wrappers/bin/bin` instead.
+
 ## `RuntimeDirectoryPreserve = "yes"`
 
 …keeps `/run/hyperhive/` (and the per-agent sub-dirs) across

docs/turn-loop.md

@@ -198,7 +198,10 @@ it as a stdio child via `--mcp-config`. The hyperhive socket name is
 
 - `send(to, body, in_reply_to?)` — message a peer (logical agent
   name), another agent, or the operator (recipient `operator`,
-  surfaces in the dashboard inbox). Optional `in_reply_to: i64`
+  surfaces in the dashboard inbox). Use `to: "<parent>"` to
+  address the agent's topology parent without knowing its label;
+  the broker resolves the sentinel at delivery time (falls back
+  to `operator` for root agents). Optional `in_reply_to: i64`
   links this message to a prior message id for thread rendering
   in the dashboard message flow and the per-agent inbox.
 - `recv(wait_seconds?, max?)` — drain inbox messages. Without
@@ -269,8 +272,7 @@ socket at `/run/hive/mcp.sock`. Two equivalent paths:
   (use `--body -` to read body from stdin). Already on the
   container's `PATH` since the harness binary is in
   `systemPackages`. Convenient for shell-script integrations.
-  Agent-only today (refused when `HIVE_ROLE=manager`); see
-  [#693](http://localhost:3000/hyperhive/hyperhive/issues/693).
+  Works for both `agent` and `manager` roles.
 
 - **Speak the wire protocol directly** — JSON-line over the
   unix socket: `{"cmd":"wake","from":"matrix","body":"new dm