feat: add LLM SQL query assistant and dataset sampler

- ask.py: Python script to query Base dos Dados via natural language using Gemini,
  generates and executes DuckDB SQL from Portuguese questions
- ask/ (Rust): CLI companion for the SQL query assistant with system prompt
- sample_datasets.py: samples parquet files from S3 into a local DuckDB for exploration
- sample_datasets/ (Rust): CLI for dataset sampling
- context/: LLM context bundle (schemas, join keys, file tree) for query generation

sample_datasets/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "sample_datasets"
+version = "0.1.0"
+edition = "2021"
+
+[[bin]]
+name = "sample_datasets"
+path = "src/main.rs"
+
+[dependencies]
+duckdb = { version = "1", features = ["bundled"] }
+arrow = { version = "=58.0.0", features = ["prettyprint"] }
+dotenvy = "0.15"
+ctrlc = "3"
+anyhow = "1"