#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.
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