docs: add noisy server troubleshooting

[haosenwang1018]

Summary

• add a README troubleshooting section for noisy MCP servers
• explain why LSP wrappers / watcher-heavy servers may log to stderr even when tools work
• point users to mcp-cli --debug / MCP_DEBUG=1 for intentional stderr inspection

Problem

Issue #3 shows a config that works but produces a large amount of stderr noise from mcp-language-server and related wrapped runtimes. Without troubleshooting guidance, users can read that output as a CLI failure even when tool discovery succeeds.

Changes

• add a dedicated README troubleshooting section for noisy MCP servers
• describe common categories of startup/shutdown stderr noise
• explain how to isolate one server and when to treat the logs as server-side chatter rather than a broken CLI config

Fixes #3

Validation

• git diff --check