Timeline: a team-health endpoint (#357) #567

What

#357 wants smoothness visible and actionable. The timeline window draws the raw numbers, but nothing turns them into a signal a person or a routine can act on.

Expected behavior

• timeline/health (read-only) reads the same window and returns, per Bot lane: turns, errors, errorRate, busyShare, running, waiting (unanswered requests) and short flags (errors, busy, idle, waiting), plus a team ok and the unhealthy ids.
• No configuration; body { windowMs?, to? } like timeline/window.

Change scope

New plugins/timeline/src/health.js (pure buildHealth), plugins/timeline/src/timeline.js (the endpoint), a case in plugins/timeline/test/health.test.js, docs/subsystems/timeline.md. Plugin only; reads only what the window already carries.

Why

Observability becomes a decision, not a chart (#357), and a routine can read ok/unhealthy without parsing the view.