docs(mcp): document LITELLM_ENABLE_STDIO_MCP opt-in for stdio servers

[devin-ai-integration]

Summary

Docs for the proxy change that turns stdio MCP transport off by default (BerriAI/litellm branch litellm_disable_stdio_mcp_by_default). A stdio MCP server runs a command on the proxy host, and the command allowlist does not stop python -c ... or npx <any package>, so leaving stdio on turns any path to proxy admin into code execution on the host. The proxy now refuses stdio servers unless the operator sets LITELLM_ENABLE_STDIO_MCP=true in the process environment, and that key is ignored under environment_variables in config or the DB

docs/mcp.md gets a warning admonition at "Add STDIO MCP Server" that explains the risk, the opt-in, and the stdio-to-HTTP bridge alternative. The other stdio setup spots link back to it. docs/proxy/config_settings.md adds the LITELLM_ENABLE_STDIO_MCP row, which the litellm test_env_keys doc check needs, and notes that LITELLM_MCP_STDIO_EXTRA_COMMANDS only applies once stdio is enabled. mcp_config_reference.md, security_owasp_llm_top10.md, and tutorials/claude_code_skills.md are updated to match

Merge this before the litellm PR, because its env-key documentation test checks out this repo's main

Link to Devin session: https://app.devin.ai/sessions/55147bfac539478f9080942ee7f5b155
Open in Devin Desktop: https://app.devin.ai/desktop/session/55147bfac539478f9080942ee7f5b155?variant=devin

[devin-ai-integration]

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Project	Deployment	Actions	Updated
litellm	Ready	Preview	Oct 2, 2026 3:51pm UTC

[devin-ai-integration]

Closing because BerriAI/litellm#44066 replaces the code PR and uses LITELLM_ENABLE_MCP_STDIO instead. Reopen and rename the variable if these docs help