Commit root AGENTS.md/CLAUDE.md symlinks so agents auto-load conventions
Track the root AGENTS.md/CLAUDE.md symlinks (-> .ai/AGENTS.md) instead of
generating them via `make claude`/`make codex`, so a fresh clone auto-loads
the agent guide with no setup step (matching transformers). The make targets
now only wire up skills; .ai/AGENTS.md, the contribution doc, and the PR
template are updated to match.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>