fix: agent uses bash for DuckDB queries instead of sql_execute — data shown twice

### What does this PR do?

When a user asks to query a local DuckDB file, the agent runs `duckdb demo.duckdb "SELECT ..."` via the bash tool instead of using `sql_execute`. This causes:

1. Raw duckdb CLI output shown in the Bash tool block (29 columns, unformatted, scrolls horizontally)
2. LLM re-formats the same data as markdown table in its text response
3. User sees the **same data twice** — once ugly, once formatted

### Expected behavior

The agent should use `sql_execute` for DuckDB queries so:
- The DataTable component renders the data **once** with type indicators, boolean ✓/✗, column stats
- No duplicate data display
- No raw CLI output flooding the screen

### Root cause

`sql_execute` requires a configured warehouse connection via the Python bridge. Local DuckDB files aren't auto-detected as connections. The agent falls back to bash because it's the only way to query the file.

### Proposed fix

Auto-detect `.duckdb` files in the working directory and register them as ephemeral warehouse connections for `sql_execute`. When the agent sees a DuckDB file, it should prefer `sql_execute` over `bash duckdb`.

### Screenshots

Data shown twice — raw bash output at top, LLM markdown table at bottom:


### Type of change

- [x] Bug fix
- [ ] New feature

### Checklist

- [ ] Auto-detect .duckdb/.db files as local connections
- [ ] Register ephemeral DuckDB connections in sql_execute
- [ ] Agent prompt guidance to prefer sql_execute over bash for queries
- [ ] Test with local DuckDB files