pi : turn .pi/gg into a system prompt addendum (#30762)
.pi/gg/SYSTEM.md repeated the full agent ruleset. Move it to .pi/gg/APPEND_SYSTEM.md and keep only the llama.cpp-specific rules, so it can be appended to a main system prompt with --append-system-prompt.
Drop the .pi/SYSTEM.md symlink setup from wc2wt.sh and pr2wt.sh. Worktrees no longer need it now that the folder holds an addendum instead of a full prompt.
Assisted-by: pi:llama.cpp/ggml-org/Qwen3.8-Flash-Next-GGUF (RTX 5090 32GB, DDR5 128GB)
