Add pnpm lint scripts for local prose and link checking

[tashian]

Summary

• Adds package.json with pnpm scripts for running Vale prose linting and markdown-link-check locally
• pnpm check runs both checks on files changed vs origin/main — the pre-push sweet spot
• Updates CONTRIBUTING.md with prerequisites, quick-reference script table, and corrected manual commands (adds missing --no-wrap --glob='!step-cli/reference/**' flags)

Scripts

Command	Description
pnpm check	Lint prose + check links on changed files
pnpm check:all	Lint prose + check links on all files
pnpm vale	Lint all prose
pnpm vale:changed	Lint prose in changed files only
pnpm links	Check links in all files
pnpm links:changed	Check links in changed files only

Test plan

• pnpm install succeeds (no dependencies)
• pnpm vale runs vale on all files, excluding step-cli/reference
• pnpm vale:changed lints only files changed vs origin/main
• pnpm check runs both changed-file checks sequentially

🤖 Generated with Claude Code