Document agent-protocol crate public API

[raffaelschneider]

Summary

The `zentinel-agent-protocol` crate could use better doc comments on its public API types. This makes it easier for agent authors to understand how to use the protocol.

What to document

Key types in `crates/agent-protocol/src/v2/`:

• `AgentPool` — connection pooling and load balancing
• `Decision` — agent response types (Allow, Block, Redirect, Modify)
• `AgentHandlerV2` — trait for implementing agents
• Transport types (`UdsAgentServerV2`, `GrpcAgentServerV2`)

How to contribute

1. Read the existing code and `crates/agent-protocol/docs/`
2. Add `///` doc comments to public types and methods
3. Include `# Errors` sections where applicable
4. Run `cargo doc -p zentinel-agent-protocol --no-deps --open` to preview

[xvchris]

I'll work on this