docs(lean4): add simp and simproc skill

[alok]

Summary

• add a focused lean4-simp-simprocs skill inside the unified lean4 plugin
• expand the shared simp-hygiene and simproc-patterns references with decision rules and authoring guidance grounded in the Lean community simp/simproc blog posts
• surface the new skill in the root and plugin READMEs so it is discoverable alongside the other focused Lean 4 skills

Verification

• ran bash plugins/lean4/tools/lint_docs.sh --verbose
• lint still reports existing broken anchors and metadata warnings elsewhere in the repo; this change did not introduce a new lint class