Unify AGENTS.md and CLAUDE.md into single source of truth
CLAUDE.md is now the primary file with all dev guidelines.
AGENTS.md is a symlink to CLAUDE.md so both filenames work.
Incorporated the vocabulary alignment rule from AGENTS.md into
CLAUDE.md's Development Principles section.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>