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

tools/build_all.sh

@@ -15,6 +15,11 @@ for t in tunes/*.py; do
     python3 "$t"
 done
 
+for s in tunes/specs/*.json; do
+    echo "== $s"
+    python3 tools/make_tune.py "$s"
+done
+
 echo "== verifying MIDI"
 python3 - <<'EOF'
 import sys, glob