chore: tighten up skill set cohesion

[robhowley]

• Split Base and mixins out of db/session.py into db/base.py across all skills (fastapi-init, sqlalchemy-models, alembic-migrations)
• Add naming convention to Base.metadata in fastapi-init scaffold (was only in sqlalchemy-models before)
• Add schemas/base.py with APIModel + ReadModel to fastapi-init scaffold
• Flesh out fastapi-init error handling: **context kwargs, validation handler, catch-all handler, cross-ref to fastapi-errors skill
• Align fastapi-errors skill with fastapi-init: detail field naming, class-level defaults, context logging pattern
• Add role-marker base classes (CreateModel, UpdateModel, QueryModel, etc.) to pydantic-schemas skill
• Consolidate command/batch/aggregate schema sections in pydantic-schemas
• Trim code-quality skill: fold standalone sections (migration bias, config/CI/ignore discipline, adoption) into existing rule lists
• Fix hardcoded app/ refs to {pkg_name}/ in request-correlation and sqlalchemy-models skills
• Remove redundant "canonical output requirements" section from sqlalchemy-models (duplicated the review checklist)
• Add "What's next" cross-skill pointers to fastapi-init