pi : canonicalize agent instructions via AGENTS.md

[beyonddream]

For changes to the core ggml library (including to the CMake build system), please open a PR in https://github.com/ggml-org/llama.cpp. Doing so will make your PR more visible, better tested and more likely to be reviewed.

Summary:

• make AGENTS.md the canonical repo instruction file
• symlink CLAUDE.md, .pi/SYSTEM.md, and .pi/gg/SYSTEM.md to AGENTS.md
• preserve AI disclosure and commit-tag guidance with [HARNESS] and [MODEL]
• prefer harness auto-detection and ask only when unclear

AI usage disclosure:
YES. codex:llama.cpp/gpt-5-codex

[beyonddream]

@ggerganov could I get a review for this pls ?