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.

TODO.md

@@ -75,7 +75,9 @@ how often the friction bites in normal use.
 
 ## Telemetry
 
-- **Per-turn stats log**: persist one row per claude turn in a new sqlite table on the per-agent state dir (or the host broker DB, indexed by agent). Columns: `started_at`, `ended_at`, `duration_ms`, `model`, `input_tokens`, `output_tokens`, `cache_read_input_tokens`, `cache_creation_input_tokens`, `tool_call_count`, `tool_call_breakdown` (JSON: `{Read: 12, Bash: 3, ...}`), `bytes_streamed`, `wake_reason` (recv'd message / reminder / operator-kick / manual), `result_kind` (ok / cancelled / failed-mid-turn / compacted), `note` (e.g. failure reason). Powers: per-agent dashboards (avg turn time over time, tool-usage histogram, cost projections from token counts × model rate), debugging stuck loops (look for repeated identical wake_reason + zero tool calls), and operator-visible "this is what your spend looked like this week" rollups. Source data is already mostly in the harness's `TurnState` + the per-event bus; just needs a sink. Keep a retention sweep (host-side) so the table doesn't grow forever.
+- **Per-turn stats: host-side vacuum sweep**: the sink writes to `/state/hyperhive-turn-stats.sqlite` on each agent's state dir; needs a periodic retention sweep mirroring `events_vacuum.rs` so the table doesn't grow forever. Default keep-window: 90 days (turn-stats are denser than events but smaller per-row, ~200B each).
+- **Surface per-turn stats on the agent web UI**: badges sourced from the new sink — `open_threads` count chip, `open_reminders` count chip, "N turns today" chip, rolling tool-call histogram tooltip on the model chip. Both `open_threads` and `open_reminders` are already columns on every row; the badge just reads the latest. The richer histograms read across rows.
+- **Stats UI on the main dashboard**: per-agent rollups (avg turn duration, tokens-since-boot, top 5 tools) on the container row. Same data source, host-side aggregation query.
 
 ## Bugs