Proxy federated SQL across multiple SQLite or DuckDB databases

[shah]

Implement a “serverless” way to execute federated SQL across one or more SQLite and/or DuckDB databases without spawning per-database servers.

The core idea is to treat the local/discovered databases as queryable sources and generate a temporary SQL script that:

1. ATTACHes each discovered database under a deterministic schema name (based on pattern, filename, or catalog identity).
2. Optionally applies guardrails (read-only mode, deny DDL/DML by default, allowlist statements) to make execution “proxy safe”.
3. Executes a user-supplied query (or a file of queries) against the attached schemas, letting SQLite/DuckDB handle the cross-database proxying via ATTACH semantics through either direct SQL or an MCP or "Skills".

Scope to explore:

• Discover databases (SQLite .db/.rssd.db/.sqlpage.db and DuckDB .duckdb) using existing discovery patterns.
• Define schema naming rules to avoid collisions and keep names stable across runs (using same proxy prefixing pattern).
• Generate an SQL file that contains the ATTACH statements plus the user’s query block.
• Generate an MCP that can access the data via AI assistants
• Support running the generated script through sqlite3 and/or duckdb CLI (or via the yard runtime) with consistent output modes (table/json/csv).
• Restrict queries to specific tenants or other "groupings" ("scoping")

Add safety defaults:

• Read-only by default (for SQLite, use immutable/query-only options where possible; for DuckDB, enforce read-only connection when feasible).
• Block or warn on DDL/DML unless an explicit flag is provided.
• Prevent writing temp files into the attached DB directories unless explicitly enabled.

Acceptance criteria:

• Can discover multiple databases and attach them under predictable schema names using same identifiers as proxy pass rules.
• Can use either naming or tokenization to group by tenants or other scopes
• Can run a query that references one attached database, or joins across multiple attached schemas, without starting any server.
• Generated SQL (ATTACH + query) can be emitted to a file for inspection/reproducibility.
• Safety defaults prevent accidental modification of source databases unless explicitly overridden.