Rename game from Whack Hare! to Nyuller

All references updated:
- src/build.sh: PRG=nyuller.prg, D64=nyuller.d64, disk name 'ny',
  all grep/pgrep patterns updated, all comments updated
- All 10 header files: include guards WHACK_HARE_* → NYULLER_*
- C source comments: 'Whack Hare!' → 'Nyuller'
- README.md: updated build command output path
- GAME.md: title updated
- tasks.md: all whack_hare.prg → nyuller.prg
- src/AGENT_CONTEXT.md: project name updated
- tools/convert_screens.py: project name updated

Build artifacts renamed: whack_hare.* → nyuller.*, whack.d64 → nyuller.d64

Note: title screen image (source_images/screen_title.png) still shows
'WHACKED' logo — that's a visual asset, not a code reference.

tools/convert_screens.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python3
 """Convert a C64 game source PNG into multicolor-bitmap .bin + .attr files.
 
-Phase 1 of the "Whack Hare!" C64 project. See tasks.md for the full spec.
+Phase 1 of the "Nyuller" C64 project. See tasks.md for the full spec.
 
 The C64 multicolor bitmap mode (VIC-II $D011 BMM=1, $D016 MCM=1) is a
 160x200-pixel, 2-bits-per-pixel, 4-colors-per-4x8-cell format. The