docs: rename demo-midi.md → midi-demo.md and add full MIDI domain reference

- Rename docs/demo/demo-midi.md → docs/demo/midi-demo.md to match the HTML artifact - Add docs/reference/midi-domain.md: canonical reference for all 31 MIDI semantic porcelain commands with full flag tables, JSON schemas, and type definitions - Expand docs/reference/cli-tiers.md MIDI section from 11 to 31 commands, grouped by category (notation, harmony, rhythm, structure, history, multi-agent, transformation, invariants) - Update docs/README.md: add MIDI/Code domain reference links to navigation table and directory map - Update docs/demo/README.md: fix filename reference and expand command list - Update README.md and docs/reference/code-domain.md: fix stale demo-midi.md links