Wiring up the MCP plumbing

A lighter fortnight by commit count, but meaningful connections got made. The GIMP helper picked up MCP communication in Rust and landed an initial LLM-to-GIMP integration through the tool pipeline — the first time a companion tool had a working bridge to the model layer. Blender helper continued iterating on model selection and behaviour. On the main app side, the system prompt and model dropdown were improved alongside documentation updates.