feat(health): System Health in three groups, a score that sees lag; admin user detail

Dashboard → System Health is grouped into AgentHub, Machine and Agent
activity. The machine (GET /api/host, every signed-in user) is drawn by
components/health/HostHealth.tsx: host card beside the CPU/memory history,
then CPU (threads as compact cells + temperatures/GPUs), disks & network,
busiest processes, and per-user tiles. Concurrency bars each show their own
agent type on hover.

The Health Score (client/src/lib/healthScore.ts) is now 100 minus what each
check costs, between an amber and a red threshold: event-loop p99
(server/lib/loop-delay.js, shared with /api/metrics), Linux PSI pressure for
CPU, memory and disk I/O (/api/host `pressure`; load per core without PSI),
RAM, the database's disk, V8 heap, hooks silent while agents work, and agent
errors in the last 24 h. All-time success rate, API errors and the transcript
cache hit rate no longer count. /api/settings/info adds server.event_loop and
activity {last_event_at, working_agents, agents_24h}.

Administration: a user row opens its detail in place (GET
/api/users/:id/details — spend, account pool share, live panes, workspaces,
newest sessions); the server panel is gone, member tmux folded into the table;
the Sessions list shows each row's owner in server mode.

server/index.js

@@ -1332,6 +1332,8 @@ if (require.main === module) {
         require("./lib/omp-share")
           .start()
           .catch((err) => console.warn("omp share failed to start:", err.message));
+        // System Health's CPU/memory history (in memory; Linux only, inside).
+        require("./lib/host-stats").startHistory();
       })
       .catch((err) => {
         console.error(listenFailure(err, PORT));