feat(db): add core schema -- profiles, positions, education, skills
Drizzle ORM schema tables mapping to id.sifa.profile.* lexicons:
- profiles: professional identity (did PK, handle, headline, location, etc.)
- positions: work history (composite PK did+rkey, FK to profiles with cascade)
- education: academic history (composite PK did+rkey, FK to profiles with cascade)
- skills: professional skills (composite PK did+rkey, FK to profiles with cascade)
Includes initial migration (0000) and schema tests.