feat: Add generate-from-db command for DB-native skill generation pipeline

Reads patterns from the database, generates skill content per application
pattern category, and upserts into the skills table with pattern links.
Supports --dry-run, --write-files, and --category options. Includes 24
tests covering all branches (create, update, unchanged, locked, errors).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: PaulJPhilp