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.

README.md

@@ -32,7 +32,7 @@ git submodule update --init --recursive
 
 ```sh
 cd src
-./build.sh            # compile main.c → build/whack_hare.prg (-O1)
+./build.sh            # compile main.c → build/nyuller.prg (-O1)
 ./build.sh -e         # run in oscar64's built-in emulator (headless, fast)
 ./build.sh -p         # PLAY: launch VICE in background (see build.sh --help)
 ./build.sh -v         # run in VICE x64 (foreground, blocks terminal)