Document agent lifecycle

[CaioHenriquexD]

Adds a simple explanation of the agent execution

---

Note

Introduces a concise lifecycle overview for agents.

• Adds docs/AGENT_LIFECYCLE.md outlining stages: initialization, runtime execution, and graceful shutdown
• Notes how understanding the lifecycle aids debugging and extension

^{Written by Cursor Bugbot for commit 99b5b41. This will update automatically on new commits. Configure here.}