turn_stats: per-turn analytics sink

new sqlite table at /state/hyperhive-turn-stats.sqlite on each
agent's state dir. one row per claude turn captures identity
(model, wake_from, result_kind), timing (started/ended_at,
duration_ms), cost (input/output/cache_read/cache_creation token
counts), behaviour (tool_call_count + per-tool breakdown JSON),
and post-turn snapshot metrics (open_threads_count,
open_reminders_count).

wire additions:
- AgentRequest/ManagerRequest::CountPendingReminders +
  Broker::count_pending_reminders_for(agent)
- Bus::observe_stream + take_tool_calls — pumps the existing
  stdout stream-json, picks out tool_use blocks, accumulates per
  turn. bin loops fold the breakdown into each row.
- TurnStats::open_default + TurnStatRow + record() — best-effort
  inserts; failures log + don't block the harness.

both ag3nt and m1nd bins capture started_at + duration via
Instant::elapsed, fetch open-thread + reminder counts from
hive-c0re via the existing socket (post-turn, best-effort), and
record one row at turn_end. record_kind splits ok / failed /
prompt_too_long; failures carry the error message in note.

todo entries for host-side vacuum sweep + reading the table back
into agent/dashboard badges.

hive-ag3nt/src/mcp.rs

@@ -41,6 +41,7 @@ pub enum SocketReply {
     Recent(Vec<hive_sh4re::InboxRow>),
     Logs(String),
     OpenThreads(Vec<hive_sh4re::OpenThread>),
+    PendingRemindersCount(u64),
 }
 
 impl From<hive_sh4re::AgentResponse> for SocketReply {
@@ -54,6 +55,9 @@ impl From<hive_sh4re::AgentResponse> for SocketReply {
             hive_sh4re::AgentResponse::Recent { rows } => Self::Recent(rows),
             hive_sh4re::AgentResponse::QuestionQueued { id } => Self::QuestionQueued(id),
             hive_sh4re::AgentResponse::OpenThreads { threads } => Self::OpenThreads(threads),
+            hive_sh4re::AgentResponse::PendingRemindersCount { count } => {
+                Self::PendingRemindersCount(count)
+            }
         }
     }
 }
@@ -70,6 +74,9 @@ impl From<hive_sh4re::ManagerResponse> for SocketReply {
             hive_sh4re::ManagerResponse::Recent { rows } => Self::Recent(rows),
             hive_sh4re::ManagerResponse::Logs { content } => Self::Logs(content),
             hive_sh4re::ManagerResponse::OpenThreads { threads } => Self::OpenThreads(threads),
+            hive_sh4re::ManagerResponse::PendingRemindersCount { count } => {
+                Self::PendingRemindersCount(count)
+            }
         }
     }
 }