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.

src/helloworld.c

@@ -2,9 +2,8 @@
 //
 // Builds a .prg that prints "Hello World" on the C64 text screen and exits.
 //
-// Build:  ../scripts/build.sh
-// Run:    x64 helloworld.prg   (or use ../scripts/build.sh -e to run in
-//                              the built-in oscar64 emulator)
+// Build:  make compile
+// Run:    make run
 
 #include <stdio.h>