Builds the dashboard UI on the /api/stats-hive endpoint: a new ST4TS
tab showing swarm totals (active agents, turns, tokens, labelled est
cost), a busiest-first per-agent table, and a model-mix bar list.
Plain tables/CSS bars — no chart lib in the dashboard bundle; per-agent
trend charts stay on each agent's own /stats page. Fetched on tab
activation + window change (pull-only, no SSE).
Also adds conn.busy_timeout(500ms) in hive_stats read_agent per review:
turn_stats is rollback-journal, so a read landing mid-INSERT would hit
SQLITE_BUSY and silently drop that active agent — wait the write out.
#1424 P2 backend. New hive_stats module aggregates every agent's
turn-stats.sqlite read-only (reusing Coordinator::kept_state_names +
agent_harness_dir, skipping missing/unreadable dbs) into swarm totals,
a busiest-first per-agent rollup, swarm model mix, and a labelled USD
cost estimate (rough model->price table; can move to a nix option
later). Exposed as GET /api/stats-hive?window=. Dashboard UI follows.
