Adding mcp registry ASI04 challenge

[syedDS]

Fulfillment of #33

[syedDS]

ASI04: Insecure MCP (SSE) Registry Switch Challenge

Adds a new CTF challenge for detecting MCP registry manipulation and lack of provenance verification.

Files added:

finbot/ctf/definitions/challenges/mcp_security/insecure_registry_switch.yaml - Challenge definition (advanced, 300pts, OWASP LLM05/LLM07)
finbot/ctf/detectors/implementations/mcp_registry_switch.py - Detector with real-time (check_event) and on-demand (check_aggregate) detection by scanning vendor records and events for injected MCP config patterns
Files modified:

finbot/ctf/detectors/registry.py - Register new detector
.gitignore - Exclude challenge solution files

[saikishu]

This will be a great addition. I will be holding off on merging till we get our first MCP integrated into core codebase. After which we can test the attack path and assumptions.