gantry: rework designs/gantry-detailed-design.md

From PR #211 review. The design doc is a mix of agentic planning instructions, architecture, and design. The agentic artifacts need to be removed and the document restructured to be useful as a human-readable reference/design document.

Start with designs/gantry-detailed-design.md and review PR #211 for the context behind this issue. Remove the agentic planning artifacts and restructure the document so it serves as a human-readable architecture and design reference. Done means the mixed planning instructions are gone and the document clearly presents the design.

Written by the indexing model from the issue text.