rename body skill to coding, deduplicate coder dev guidelines
The body skill and coder agent had ~240 lines of identical content —
project structure, coding standards, testing, and environment docs
were inlined in both places with drift risk.
- Rename muse/body/ → muse/coding/ — clearer name, "coding" is the
skill any Claude Code session loads for solstone dev context, while
"coder" is the cogitate agent that orchestrates staged dev workflow
- Remove inlined dev guidelines from coder.md (lines 72-309) and
replace with a short reference to the coding skill
- coding skill is now the single source of truth for dev guidelines
- Update test to verify new architecture (coder references coding
skill, coding skill + reference files exist)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>