docs: clarify stateful server limitations

[haosenwang1018]

Summary

• document that mcp-cli is request-oriented, not a persistent interactive session runner
• explain why browser/session-heavy MCP servers are a poor fit for separate mcp-cli call ... invocations
• point users toward persistent MCP clients/runtimes for durable browser state

Problem

Issue #21 reports that stateful servers such as chrome-devtools-mcp are hard to use through repeated mcp-cli call ... commands because browser/session state is not guaranteed to behave like one long interactive session.

Changes

• add a README note under connection pooling that the daemon is an optimization, not a persistent interactive session model
• add a dedicated Stateful server limitations section explaining which server types are affected and why
• recommend native/persistent MCP clients for browser/session workflows that need durable state across steps

Fixes #21

Validation

• git diff --check