feat: add Northwind sample DB setup and fix PostgreSQL schema defaults

[Adoit]

• Add scripts/setup-northwind.sh to download northwind.sql and spin up postgres via docker-compose-northwind.yml (port 5433)
• Add docker-compose-northwind.yml for the Northwind sample database
• Add docs/testing-with-northwind.md with end-to-end test walkthrough
• Add docs/opencode-northwind-example.png showing OpenCode NL query result
• Add AGENTS.md and opencode.json to guide OpenCode away from codebase scanning and toward uv run oda query for data questions
• Add memory/order-revenue-calculation.md as first memory entry
• Fix cli_schema: tables/describe/sample/profile defaulted to schema 'main' for all dialects; now resolves to 'public' (PostgreSQL), database name (MySQL), or 'main' (SQLite) from _get_inspector()
• Fix data-agent.md.tmpl: replace all bare 'oda' with 'uv run oda' so OpenCode can invoke commands without oda on PATH
• Add IMPORTANT block to template directing agent to query data directly rather than scanning source files
• Add tests for oda tables default schema resolution (SQLite and PG)
• Remove dead default 'main' from _parse_table_arg signature
• gitignore docker/northwind/northwind.sql (downloaded at runtime)