Global hooks config + Pi agent support

[elyase]

Summary

• switch hooks to a global .ruler/hooks.json source with optional per-agent overrides (mirrors skills)
• add Pi Coding Agent support (AGENTS.md + native skills directory .pi/skills)
• extend skills propagation + gitignore to include .pi/skills
• update docs and tests for hooks + skills

Testing

• npm ci
• npm run lint
• npm test
• npm run build

[intellectronica]

Hi @elyase, thanks for the contrib. I'm going to have to review it a bit more thoroughly, since it's a new feature.
Also, it should be tagged as experimental in the CLI and in the README.

In the future, I'd like to suggest that you file an issue and kick off a discussion before adding completely new functionality in a PR. I try to be very lightweight with project management, but it doesn't mean I'm going to merge every PR. Thanks!