UNION forces a sort-and-dedupe pass; UNION ALL concatenates. On large
result sets the difference is often an order of magnitude. In most real
queries people write UNION out of habit when UNION ALL is correct -
either because the two sides are already disjoint (e.g. different tables,
different date ranges) or because duplicate rows aren't possible given the
schema.

A W011 rule warns on every UNION (not UNION ALL) so the author has
to actively justify the dedupe cost.

Should fail

SELECT id FROM orders_2024
UNION
SELECT id FROM orders_2025;

Should pass

SELECT id FROM orders_2024
UNION ALL
SELECT id FROM orders_2025;

Implementation hints

• Mirror W005 subquery-in-where in sql_guard/rules/warnings.py - it's
  a single-statement multiline rule.
• Regex: r"\bUNION\b(?!\s+ALL\b)"
• Severity: warning.
• Add to ALL_RULES in sql_guard/rules/__init__.py.
• Test: copy the shape of test_rules.py::test_missing_limit - one
  triggering input, one non-triggering input.

Estimated LOC: ~25 code + ~15 test.