chore: add contributing guide, update CI workflows and Makefile

[beer-1]

Summary

• Add CONTRIBUTING.md with prerequisites, commit convention, branch naming, build/test/lint/fmt commands, and code generation guide
• Update spellcheck.yml: switch to pip install codespell, add push trigger on main, update actions/checkout to v5, expand skip patterns
• Add modernize-go.yml workflow for weekly go fix ./... with auto PR (excludes generated types/bcs.go)
• Restructure Makefile fmt/lint targets: make lint (clippy), make fmt (rust-fmt + go-fmt + move-fmt), with individual make rust-fmt, make go-fmt, make move-fmt

Test plan

• make lint runs clippy successfully
• make fmt runs all formatters (rust, go, move)
• make rust-fmt / make go-fmt / make move-fmt work individually
• Spellcheck workflow triggers on PR and main push
• Modernize Go workflow can be triggered manually

🤖 Generated with Claude Code