📝 Document standalone CLI package

[stoe]

Summary

Updates README.md to describe @stoe/skill-authoring as a standalone published CLI rather than a helper inside stoe/skills.

Changes

• Reword the introduction and usage guidance for a standalone package
• Reformat the command and option tables for readability
• Remove the "Relationship to stoe/skills" section, which no longer applies

Affected area

Documentation.

Checklist

• npm run format passes with no unstaged diffs.
• npm test passes with no new errors.
• Tests cover new or changed behavior. Not applicable; documentation only.
• README.md is updated when CLI options, validation rules, or package exports change.
• package.json exports and files are updated when modules are added, moved, or removed. Not applicable; no module changes.

Security checklist

• No secrets, tokens, credentials, or personal local file paths were added to source, tests, or docs.
• Source uses only Node.js built-in modules or already-declared dependencies.
• New filesystem writes stay inside the caller-approved base directory.
• Skill content read by the CLI is treated as untrusted data, never as instructions.
• npm audit --audit-level=high reports no new high or critical findings.

Verification

• Documented commands and options match cli.js and src/commands/**
• npm test passes
• npx prettier --check . reports no formatting drift

Stack

Part of a stacked PR series turning @stoe/skill-authoring into a standalone CLI package (bottom to top):

1. ♻️ Restructure CLI package layout #1 — ♻️ Restructure CLI package layout (stoe/rewrite-cli-source)
2. ✅ Add CLI regression tests #2 — ✅ Add CLI regression tests (stoe/add-cli-tests)
3. 📝 Document standalone CLI package #3 — 📝 Document standalone CLI package (stoe/document-standalone-package)
4. 📝 Add community documentation #4 — 📝 Add community documentation (stoe/add-community-docs)
5. 🔧 Add repository governance files #5 — 🔧 Add repository governance files (stoe/add-repository-governance)