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.

GAME.md

@@ -1,4 +1,4 @@
-# GAME.md — *Whack Hare!*
+# GAME.md — *Nyuller*
 
 A two-player reaction-time duel for the Commodore 64. Two players stand
 back-to-back, take ten paces, and the first one to hit fire when the