Document common agent lifecycle states and their meaning

## Description
AgentKit agents move through several internal lifecycle states during execution, but these states are not clearly documented. Developers may need to infer meaning from logs or source code.

### Problem Description
Without clear documentation, developers may be unsure:
- when an agent is considered idle, running, or completed
- how to interpret intermediate states during retries or waiting
- which states are terminal vs recoverable

This makes debugging and monitoring agent behavior harder than necessary.

## Proposed Solution
Add a short documentation section that lists common agent lifecycle states, explains what each state represents, and describes typical transitions between them.

### Acceptance
The documentation clearly defines agent lifecycle states and their meaning. Developers can reason about agent execution without inspecting internal code. No changes are made to runtime behavior.