Document Non-Obvious Conventions for AI Coding Agents

Use your config file to state critical, non-obvious conventions the AI might miss from looking at a single file: named vs. default exports, standard API response shapes, migration file locations, and environment variable validation rules.

## Critical Project Conventions
- Exports: Always use named exports; never use default exports.
- API Responses: Return standard { data, error, meta } JSON shapes from all handlers.
- Migrations: Place all schema migrations in packages/db/migrations/.
- Env Validation: Register all new environment variables in src/envSchema.ts before use.