Document custom MCP server integration

[josephsenior]

Problem

Grinta supports MCP (Model Context Protocol) servers, but there's no guide explaining
how to connect a custom MCP server or build one that works with Grinta.

Proposed solution

Add a docs/MCP_INTEGRATION.md guide covering:

1. What MCP is — brief explanation with link to the spec
2. Connecting an existing MCP server — configuration in grinta.toml
3. Building a custom MCP server — minimal example (e.g., a server that exposes
   a database query tool)
4. Testing the integration — how to verify tools are discovered and callable
5. Troubleshooting — common issues (auth, transport, tool schema validation)

Include a working example in examples/custom-mcp-server/ with:

• A minimal Python MCP server (~50 lines)
• A grinta.toml snippet to connect it
• A test task that uses the custom tool

Acceptance criteria

• docs/MCP_INTEGRATION.md written and linked from main README
• Working example in examples/custom-mcp-server/
• Example tested in CI