Add MCP framework remediation guide

[mindbomber]

Summary

• add a framework remediation guide index with an MCP guide as the first concrete framework slice for Per-framework vulnerability guides — remediation examples for LangChain, CrewAI, MCP, OpenAI, Vercel AI #121
• document MCP before/after fixes for input schemas, filesystem scoping, package pinning, shell-tool gating, and redacted tool-call audit events
• link the new guide from the documentation index and framework guide, with a focused docs regression test

Tests

• npm test -- tests/unit/framework-remediation-docs.test.ts
• npm run lint
• git diff --check

Part of #121.