feat: agent health scores — per-agent sub-metric badges

[JLRansom]

Summary

• Adds per-agent health sub-metrics (completion rate, throughput trend, error density, idle time) derived from taskRuns history with zero new DB tables or API routes
• Surfaces metrics as a compact four-dot colour-coded badge on every agent row in the dashboard, updating with each SSE broadcast via a 30-second server-side TTL cache
• Ships sub-metrics first (phased approach) — composite 0–100 score deferred until weights are validated with real usage data

Architecture

lib/health.ts — pure computeHealthMetrics(taskRuns, now?) function; injectable now makes every calculation fully testable without mocking Date.now(). Uses a 7-day rolling window (ROLLING_WINDOW_MS), a MIN_RUNS_THRESHOLD = 5 sparse-data guard, and simple linear regression for week-over-week throughput trend.

lib/health-cache.ts — module-level Map singleton (same pattern as broadcast.ts) with configurable TTL. Health metrics are never computed inside the SSE hot path — getStreamPayload() serves cached values and only recomputes on cache miss or expiry.

components/AgentHealthBadge.tsx — four coloured dots (C = Completion, T = Trend, E = Errors, I = Idle) with threshold-based green/yellow/red colouring and a hover tooltip showing actual values. Renders nothing on missing metrics; shows "Gathering data…" when hasEnoughData is false.

Failure mode handling

• Agent deleted mid-broadcast → agentHealth?.[agent.id] returns undefined → badge renders nothing
• Missing/malformed agentHealth field in older payload → StreamPayload.agentHealth is optional — existing clients degrade gracefully
• Sparse history (< 5 runs in window) → explicit "Gathering data…" label, no misleading zeros

Files changed

File	Action
lib/types.ts	Add AgentHealthMetrics interface; extend StreamPayload.agentHealth?
lib/db/repositories/taskRunRepo.ts	Add dbGetTaskRunsByAgent + dbGetRecentTaskRunsByAgent
lib/health.ts	New — pure metrics calculation
lib/health-cache.ts	New — 30-second TTL cache
lib/store.ts	Wire health cache into getStreamPayload()
components/AgentHealthBadge.tsx	New — four-dot badge component
components/AgentRow.tsx	Add Health cell
components/AgentList.tsx	Add "Health" column header
__tests__/unit/health.test.ts	New — 13 unit tests (deterministic, injected now)
__tests__/unit/health-cache.test.ts	New — 9 tests (TTL, DB reads, invalidation)

Test plan

• npm run test:run — all 156 tests pass
• Sparse-data guard: agents with < 5 runs in window show "Gathering data…" badge
• Sufficient-data path: agents with ≥ 5 runs show four coloured dots
• Hover tooltip shows Completion: X% · Trend: ±Y tasks/wk · Errors: Z% · Idle: Nm
• Deleting an agent removes their row and badge with no JS errors
• SSE payload includes agentHealth map; older clients without field ignore it gracefully

Depends on: PR #26 (fix/analytics-status-classification) — merge that first.

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