Tighten AGENTS.md and extract LLM provider checklist to skill
- Tighten all sections (279 -> 159 lines) without dropping rules.
- Reorganize: git rules moved next to issues/PRs.
- Defer contributor gate details to CONTRIBUTING.md.
- Replace stale npx tsx test command with node (strip-only mode); add ./test.sh + e2e warning.
- Releasing: explicit WebAuthn briefing step, no bash timeout, stop on partial publish failure.
- Move LLM provider checklist to .pi/skills/add-llm-provider.md.