- Import LibSQLClient type for seedBuiltinGroups parameter typing
- Add DDL constants for 5 new tables: skill_groups, skill_group_members,
skill_group_inheritance, agent_skill_groups, agent_skills
- Add BUILTIN_GROUPS constant with 5 entries (pm-essentials, engineer-core,
frontend, backend, creative)
- Add seedBuiltinGroups() using INSERT OR IGNORE for idempotent seeding
- Extend getSkillRegistryDb() to execute all 5 new DDL statements and seed