AGENTS.md: correct rendering docs — FluidR3 is MIT (size, not licence), add OPL3 route

AGENTS.md

@@ -72,8 +72,15 @@ to ticks at 480 tpq.
 
 ## Rendering
 
-Needs `timidity` + a GM soundfont (e.g. FluidR3_GM.sf2, not bundled —
-licensing). Debian's timidity: use `-Ow -s 44` (no `-O2` mode).
+Two routes, both verified:
+- **Sample-based**: `timidity -Ow -s 44 in.mid -o out.wav` + a GM soundfont.
+  FluidR3 is MIT-licensed and one command away: `apt install fluid-soundfont-gm`
+  (148MB; lighter: `timgm6mb-soundfont`, ~6MB). Not bundled in-repo because of
+  size, not licence. Debian's timidity: use `-Ow -s 44` (no `-O2` mode).
+- **OPL3 FM (authentic DOS sound, no soundfont)**: `./tools/opl/build.sh`
+  once (needs cmake+g++, fetches libADLMIDI), then `./tools/render_opl.sh`.
+  `OPL_MODEL=` env picks a game-driver volume model: 3 DMX/Doom (default),
+  4 Apogee/Duke Nukem 3D, 2 id-style, 8 AIL, 10 HMI. Output is normalised.
 
 ## Known gotchas