Extract shared Node library (lib.js)

[sungjunlee]

Summary

Extract duplicated pure functions from sync-pull.js and sprint-init.js into scripts/lib.js.

Motivation

• slugify() is identical in both files (and tested identically in both test files)
• escapeYaml() is a general-purpose utility only in sync-pull.js but useful elsewhere
• DRY violation: any change must be made in two places

Scope

1. Create scripts/lib.js with shared functions: slugify, escapeYaml
2. Update sync-pull.js and sprint-init.js to require('./lib')
3. Create scripts/lib.test.js with consolidated tests
4. Remove duplicate tests from individual test files

From engineering review

Covers review finding #5 (slugify duplication).

[sungjunlee]

Done in PR #18 — lib.js extracted with slugify, escapeYaml