add event and observability reference

[blindzero]

IdLE emits structured runtime events (RunStarted, StepCompleted, StepFailed, etc.).
To improve debuggability and integration, a reference describing these events and their payloads is required.

Scope

• List all emitted engine events
• Describe event lifecycle and ordering
• Document payload structure
• Explain how events are consumed (sinks / handlers)

Out of scope

• No new event types
• No behavior changes

Acceptance Criteria

• A document exists under docs/reference/events.md
• Events are grouped by lifecycle phase
• Payload fields are documented
• Linked from docs/index.md