✅ Add CLI regression tests

[stoe]

Summary

Adds a Node.js test suite covering the CLI entry point, both commands, the core helpers, and the validation rules, then wires it into npm test so regressions surface in CI.

Changes

• Add 19 tests across test/cli.test.js, test/commands/**, test/core/**, and test/validate/**
• Cover argument parsing, init scaffolding, validate reporting, frontmatter parsing, filesystem helpers, logging, and the security rules
• Run node --test test/**/*.test.js as part of npm test

Affected area

Tests, npm test script.

Checklist

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

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. Tests write only to mkdtemp directories.
• 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

• npm test reports 19 passing tests across 8 suites, 0 failures
• npx prettier --check . reports no formatting drift
• npm audit --audit-level=high finds 0 vulnerabilities

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)