Make the AI assistant more effective by providing focused, actionable
instructions instead of a monolithic reference document.

- Slim down CLAUDE.md from 500+ lines to 63 lines
- Extract coding practices, testing conventions, commit conventions,
  and LSP fallbacks into path-scoped .claude/rules/ files so they
  load only when relevant files are being edited
- Add 4 build skills (/build, /test, /lint, /ci) using sub-agents
  to keep verbose output out of the main context window
- Add build warning hook to nudge toward skill usage
- Remove outdated .github/copilot-instructions.md (superseded by
  CLAUDE.md and .claude/rules/)
- Narrow .gitignore build/ pattern to /packages/**/build/

- Use [[ ]] instead of [ ] in block-build.sh for idiomatic bash and robustness
- Fix npm command regex to catch both 'npm test' and 'npm run test' variants
- Add language specifier (text) to code blocks in skill markdown files for linting