Add research prompts for LLM tool-calling & MCP

Add two spike research prompts: SPIKE_618 and SPIKE_619. SPIKE_618 documents a deep research prompt for designing LLM function/tool-calling in Taskdeck (architecture, provider comparisons, tool inventory, multi-turn flow, mock provider strategy, cost/latency model, and phased implementation). SPIKE_619 documents a deep research prompt for an MCP server to expose Taskdeck resources to external agents (protocol deep dive, .NET implementation options, resource/tool URIs, auth/permission models, proposal lifecycle, hosting recommendations, and prototype scope). These files provide guidance and deliverables to drive implementation decisions and prototypes.

Author: Chris0Jeky