feat(sacp-cookbook): create dedicated cookbook crate with comprehensi…

[nikomatsakis]

…ve examples

• Create sacp-cookbook crate with dependencies on sacp, sacp-conductor, sacp-rmcp, and sacp-tokio so all examples can compile as real doctests
• Add one_shot_prompt entry showing the simplest client pattern
• Simplify per_session_mcp_server to focus on on_proxy_session_start callback
• Add rmcp example to global_mcp_server (now compiles, was previously ignored)
• Update sacp cookbook.rs to redirect to sacp_cookbook crate
• Update sacp lib.rs to reference external cookbook crate