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.

TODO.md

@@ -3,21 +3,6 @@
 Pick anything from here when relevant. Cross-cutting design notes live in
 [CLAUDE.md](CLAUDE.md); high-level project intro in [README.md](README.md).
 
-## Turn loop
-
-- **`recv` with no `wait_seconds` should return immediately.**
-  Today omitting the argument falls through to the 30s
-  default long-poll (`RECV_LONG_POLL_DEFAULT` in
-  `hive-c0re/src/agent_server.rs`); a manager that wants a
-  cheap "anything in the inbox right now?" peek has to
-  explicitly pass `wait_seconds: 0`. Flip the semantics so
-  `None` = no sleep, returning `None` (or the empty inbox
-  shape) right away. The agent opts into the long-poll by
-  setting a positive value. Update both `AgentRequest::Recv`
-  and `ManagerRequest::Recv` handlers + the prompt language
-  in `prompts/{agent,manager}.md`. Tighten the cap (180s)
-  too — only meaningful when the agent is choosing to wait.
-
 ## Permissions / policy
 
 - **Per-agent send allow-list.** Today any agent can `send` to any