docs: add MCP client usage examples

[JackSpiece]

Closes #201

Summary

• Add examples/mcp_client_usage.py, a runnable stdio MCP client that lists Agentic Security MCP tools and can call no-argument HTTP-backed tools when the app is running.
• Add docs/mcp_client_usage.md, wire it into MkDocs navigation, and link it from the README.
• Update the existing agentic_security.mcp.client helper to launch the server with python -m agentic_security.mcp.main and list the real server tools instead of calling a nonexistent echo_tool.
• Let the MCP server read AGENTIC_SECURITY_URL so examples can target a non-default local app URL.
• Update the MCP unit test expectation to check the actual Agentic Security tool names.

Testing

• python -m py_compile agentic_security/mcp/main.py agentic_security/mcp/client.py examples/mcp_client_usage.py
• git diff --check
• python -m pytest tests/unit/test_mcp.py -q stops before collecting this test because this Termux checkout does not have the project dependency sklearn installed: ModuleNotFoundError: No module named 'sklearn'.
• I also tried a temporary MCP/httpx-only venv for a live MCP smoke test, but pip started compiling Rust build tooling (maturin) on Termux instead of using wheels, so I stopped and removed the venv to avoid wasting phone storage.

Check note

• The visible Vercel check currently reports Authorization required to deploy for this fork. I do not see a code failure in that check.