refactor: inline package name resolution during diff parsing

Optimizes package name resolution by reading package.json inline during
git diff parsing instead of in a separate enrichment pass.

Changes:
- Make parseDiff async to read package names inline
- Remove enrichWithPackageNames function (no longer needed)
- Read packageName immediately when first encountering a package
- Simplify validateChangelogs and updateChangelogs signatures
- Remove packageNames parameter (now part of PackageInfo)

Benefits:
- Single-pass processing (parse + enrich in one step)
- Simpler code flow (24 lines removed)
- Better data locality (package info complete at creation)
- Cleaner API (functions receive unified PackageChanges structure)

Test coverage maintained: 100% (339 passing tests)

Author: cryptodev-2s