Replace src/build.sh with Makefile (GNU make)

New targets:
  help (default)     — show usage (was the -help/--help equivalent)
  compile            — compile main.c → build/nyuller.prg
  run                — compile + run in oscar64 built-in emulator
  run-vice           — compile + run in VICE x64 (foreground)
  run-vice-cycle     — compile + run in VICE x64sc (cycle-exact)
  play               — compile + launch VICE x64 detached
  play-cycle         — compile + launch VICE x64sc detached
  kill               — kill any detached VICE
  clean              — remove build/ artifacts
  build/nyuller.d64  — build the .d64 disk image from the .prg

Optimization: override with 'make OPT=O3' (or O0/O1/O2/Os/g).
Default is -O1 (oscar64 default).

The Makefile builds oscar64 automatically if missing, uses
c1541 to create the .d64 with verification, and passes
-drive8type 1541 to VICE (required for autostart).

Removed src/build.sh and updated all docs (README, tasks.md,
AGENT_CONTEXT.md, GAME.md, helloworld.c) to reference make.

GAME.md

@@ -355,7 +355,6 @@ src/
 ├── draw.c / .h         # the white-screen + counter rendering
 ├── rasterirq.c / .h    # 50 Hz frame tick
 ├── notes.h             # SID frequency table
-├── build.sh            # (already exists)
 └── build/              # (gitignored) output
 ```