hive-sh4re + docs: extract broker delivery/ack/requeue prose (#717 batch 1)

docs/conventions.md

@@ -54,6 +54,43 @@ each frame carries a `seq` field for the snapshot-dedupe dance
 — change them in one place. The dashboard event vocabulary lives
 in `hive-c0re::dashboard_events::DashboardEvent`.
 
+### Broker delivery + ack cycle
+
+`AgentRequest::Recv` is the only path that delivers messages to an
+agent. Always returns a list (`Messages { messages }`) — empty when
+nothing's pending, single-pop when `max = None` (default 1, the
+single-message behaviour), batched up to `max` when caller asks for
+more (server-side cap is 32; values above clamp silently).
+`wait_seconds` long-polls for the first message; once one arrives —
+or one is already pending — the call drains up to `max` in total
+before returning, so a single `Recv` call coalesces a burst.
+
+Per-row bookkeeping inside the broker:
+
+- `delivered_at = NOW` set on every popped row.
+- Each recipient has an in-memory `unacked_ids` list of every row
+  delivered since the last `AckTurn`.
+- `redelivered = true` on a row if `RequeueInflight` resurfaced it
+  (the harness prepends a "may already be handled" hint when this
+  flag is set so the per-message warning is visible).
+
+`AgentRequest::AckTurn` closes out the in-memory list — the harness
+fires it after `TurnOutcome::Ok`, marking every message popped since
+the last ack as fully handled. Claude doesn't see this surface; it's
+strictly a harness↔broker pairing. On `TurnOutcome::Failed` the
+harness intentionally skips the ack so the unacked rows stay
+in-flight in the DB and get picked up by the next requeue sweep.
+
+`AgentRequest::RequeueInflight` is the recovery pair: fired by the
+harness exactly once at boot, before the serve loop starts. Catches
+the crashed-mid-turn / OOM-killed / container-restarted cases where
+a previous harness session popped messages but never drove them to
+a clean turn-end. Resets `delivered_at` back to NULL on every
+unacked row (so the next `Recv` pops them again), and remembers
+each id in a per-recipient in-memory set so the next `Recv` can tag
+the row with `redelivered: true`. Idempotent + cheap when there's
+nothing in flight, so the at-boot fire is unconditional.
+
 ## Async forms
 
 Dashboard + per-agent mutating forms carry `data-async`; a delegated