chore(config): unify configs/ into config/ per RSR-H11

RSR canonical layout names a single config/ directory for Nickel configs;
echidna had a config/ (with solver-manifest.toml) AND a sibling configs/
(with a single config.ncl). Merge the latter into the former and drop
the empty configs/ directory. Single-home principle restored.

Verified: no Justfile recipe, CI workflow, or build script references
configs/config.ncl.

https://claude.ai/code/session_01YPqu7gti4azBach6ZvpRFJ

Author: claude