Add DuckDB connector configuration types

[intraq-dev-ai]

Goal

Add shared DuckDB connector configuration types/defaults.

This is a small first step toward #18.

Acceptance criteria

• Define the public config shape needed by a DuckDB connector.
• Include fields for local file/database path and read-only behavior if appropriate.
• Do not add query execution in this issue.
• Keep secrets and local paths out of committed examples.