Rename MusicRGA → MidiRGA and purge all 'music plugin' terminology

The MIDI plugin is the domain; 'music' was a legacy holdover.

Class / file renames: - MusicRGA → MidiRGA throughout muse/plugins/midi/_crdt_notes.py - tests/test_music_plugin.py → tests/test_midi_plugin.py (git mv) - docs/crdt-music-rga.md → docs/crdt-midi-rga.md (git mv)

Module docstrings (muse/plugins/midi/): - "Muse music plugin" → "Muse MIDI plugin" in midi_diff, manifest, midi_merge, _invariants, entity, __init__

Domain string literals: - domain="music" → domain="midi" in tour_de_force.py, render_html.py - to_structured_delta("music") → ("midi") in op_log.py example - _ICONS["music"] key → _ICONS["midi"] in render_domain_registry.py

Agent / workspace rules: - AGENTS.md: MusicPlugin → MidiPlugin, music domain → MIDI domain, test_music_plugin.py → test_midi_plugin.py - .cursorrules: same, plus quick-reference table row updated

Tests: - test_core_merge_engine.py: test_music_plugin_isinstance → test_midi_plugin_isinstance - tests/test_crdts.py: "music plugin" comment → "MIDI plugin"

Docs: - demo-script.md: MusicPlugin.snapshot/merge → MidiPlugin.* - supercharge-plan.md: test_music_plugin_* refs → test_midi_plugin_* - muse-protocol.md, op-log.md, type-contracts.md: "music" example → "midi" - plugin-authoring-guide.md: "music": MidiPlugin() → "midi": MidiPlugin() - muse-domain-concepts, muse-variation-spec, muse-vcs: music plugin → MIDI plugin - render_domain_registry.py: MusicRGA → MidiRGA capability pill + CRDT text