Add native pi coding agent integration guide

The repo has setup guides for Claude Code, OpenCode, Cursor, Gemini CLI, Copilot, Windsurf, Antigravity, and Kiro. Pi is a terminal coding harness that supports the Agent Skills standard natively. It has no plugin system and no dedicated skill tool. It discovers skills through convention directories and loads them with the read tool.

There is currently no pi setup guide, no README section, and no AGENTS.md guidance for pi users.

I have a branch (feat/pi-native-integration at H3xKatana/agent-skills) with a proposed implementation:
- docs/pi-setup.md with install instructions and integration details
- README update adding Pi between Kiro and Codex in the native integrations section
- AGENTS.md update with a Pi section covering the read-tool difference

Happy to open a PR if this direction looks right.