recv: None = peek, positive value = opt-in long-poll

old behavior: omitted wait_seconds fell through to the 30s
RECV_LONG_POLL_DEFAULT — claude calling 'is there anything in
my inbox right now?' between actions blocked the turn for half
a minute. flip the semantics: None (or 0) returns immediately,
positive value parks up to MAX (180s, unchanged). cleaner
'peek vs wait' distinction; tool descriptions + agent/manager
prompts updated to point at the new shape.

harness's own serve loops in hive-ag3nt + hive-m1nd relied on
the old default for their inbox poll. they now explicitly pass
wait_seconds: Some(180) to opt into the full park — same
effective behavior as before, just spelled out.

retires the matching TODO under Turn loop.

docs/turn-loop.md

@@ -102,11 +102,11 @@ it as a stdio child via `--mcp-config`. The hyperhive socket name is
 - `send(to, body)` — message a peer (logical agent name), another
   agent, or the operator (recipient `operator`, surfaces in the
   dashboard inbox).
-- `recv(wait_seconds?)` — drain one inbox message. Long-polls
-  server-side; `wait_seconds` is capped at 180 (default 30 when
-  omitted). Agents use a long wait to park their turn waiting for
-  work instead of busy-looping with short polls — they wake
-  instantly when a message arrives.
+- `recv(wait_seconds?)` — drain one inbox message. Without
+  `wait_seconds` (or with `0`) returns immediately, a cheap
+  "anything pending?" peek. Positive value parks the turn up
+  to that many seconds (cap 180) — incoming messages wake
+  instantly, otherwise returns empty at the timeout.
 - `ask_operator(question, options?, multi?, ttl_seconds?)` —
   surface a question on the dashboard. Same shape as the manager's;
   answer routes back to the asker's own inbox as