Public-ready: JSON tune specs, first-class loop API, licence, AGENTS.md

- MidiFile.set_loop(): one call writes LOOP_START/END, RPR LOOP
  START/END (Reaper) and LoopBegin/End markers - every engine finds
  its dialect; reader now parses markers and the smoke test asserts
  the round-trip
- tools/make_tune.py: declarative JSON spec -> fully arranged MIDI
  with style presets (dos_adventure, n64_ambient, chiptune_racing);
  the agent-friendly front door, no Python required
- tunes/specs/: demo tune 04 'Tea Break at the Terminal' (Am/112,
  loop bars 4-20) generated entirely from JSON
- AGENTS.md: spec schema, score grammar, verification loop, looping
  conventions for game BGM
- MIT LICENSE; history audited clean of credentials

tunes/03_late_equaliser.py

@@ -128,8 +128,7 @@ def build():
 
     mf.add_track(lead); mf.add_track(harm); mf.add_track(stabs)
     mf.add_track(bass); mf.add_track(d)
-    lead.marker(bar(2), "LOOP_START")
-    lead.marker(bar(26), "LOOP_END")
+    mf.set_loop(bar(2), bar(26))
     return mf