Convert GitHub Actions CI to pixi

[jayhesselberth]

Summary

Convert CI workflows from r-lib/actions to pixi for faster builds. All R package dependencies are available on conda-forge/bioconda.

Plan

See .claude/plans/parsed-wiggling-hellman.md for the full implementation plan.

Key changes:

• Add all R package deps to pixi.toml for full conda caching
• Replace r-lib/actions with prefix-dev/setup-pixi in both R-CMD-check.yaml and pkgdown.yaml
• Single R version (conda-forge release) — no matrix
• Only rnabioco/rbitemplate needs pak (GitHub-only package)

Files to modify

• pixi.toml
• .github/workflows/R-CMD-check.yaml
• .github/workflows/pkgdown.yaml