The docs half of Event System Phase 2. Pairs with `RunicGateway/website`
`feat/events-phase-2`.
EVENTS.md gains what the runner settled: the parked step (`running` with a NULL
lease), the two success-envelope members `await: 'human'` and `holdFor`, the
answer for a run whose concurrency key is held, `n` in §L's `retry(n)`, the rule
that all three `on_failure` dispositions write the step `failed`, and the health
transition on the first retry rather than the eventual failure.
EVENTS_PLAN.md marks Phase 2 complete and records the four org-lead decisions
and the three things the build settled on its own.
Co-Authored-By: Claude <noreply@anthropic.com>
