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.py

@@ -0,0 +1,62 @@
+import duckdb
+import os
+from dotenv import load_dotenv
+
+load_dotenv()
+
+BUCKET       = os.environ['HETZNER_S3_BUCKET']
+ENDPOINT_URL = os.environ['HETZNER_S3_ENDPOINT']
+ACCESS_KEY   = os.environ['AWS_ACCESS_KEY_ID']
+SECRET_KEY   = os.environ['AWS_SECRET_ACCESS_KEY']
+
+s3_endpoint = ENDPOINT_URL.removeprefix('https://').removeprefix('http://')
+
+con = duckdb.connect('basedosdados.duckdb')
+con.execute("LOAD httpfs;")
+con.execute(f"""
+    SET s3_endpoint='{s3_endpoint}';
+    SET s3_access_key_id='{ACCESS_KEY}';
+    SET s3_secret_access_key='{SECRET_KEY}';
+    SET s3_url_style='path';
+    SET enable_object_cache=true;
+    SET threads=4;
+    SET memory_limit='6GB';
+""")
+
+schemas = [row[0] for row in con.execute(
+    "SELECT schema_name FROM information_schema.schemata WHERE schema_name NOT IN ('main', 'information_schema', 'pg_catalog')"
+).fetchall()]
+
+try:
+    with open("dataset_sample.txt", "a") as f:
+        f.write("# Dataset samples\n\n")
+
+        for schema in sorted(schemas):
+            tables = [row[0] for row in con.execute(
+                f"SELECT table_name FROM information_schema.tables WHERE table_schema = '{schema}'"
+            ).fetchall()]
+
+            for table in sorted(tables):
+                full = f"{schema}.{table}"
+                try:
+                    rows = con.execute(
+                        f"SELECT * FROM {full} USING SAMPLE 2 ROWS"
+                    ).fetchall()
+                    cols = [f"{d[0]}:{d[1]}" for d in con.description]
+
+                    f.write(f"## {schema}/{table}/\n")
+                    f.write(",".join(cols) + "\n")
+                    for row in rows:
+                        f.write(",".join("" if v is None else str(v) for v in row) + "\n")
+                    f.write("\n")
+                    f.flush()
+                    print(f"done: {full}")
+                except Exception as e:
+                    f.write(f"## {schema}/{table}/\n[error: {e}]\n\n")
+                    f.flush()
+                    print(f"error: {full}: {e}")
+
+except KeyboardInterrupt:
+    print("\nCancelled.")
+
+con.close()