todo: recv default should not sleep, agent opts into wait

TODO.md

@@ -3,6 +3,21 @@
 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