feat(lint): add linter rules for migration and naming conventions

* Implement migration safety rules:
  - MIG001: Warn on CREATE INDEX without CONCURRENT.
  - MIG002: Warn on ALTER TABLE ADD COLUMN NOT NULL without DEFAULT.
  - MIG003: Warn on ALTER TABLE ADD CONSTRAINT without NOT VALID.
* Implement naming conventions rules:
  - NAM001: Warn on non-snake_case table names.
  - NAM002: Warn on non-snake_case column names in CREATE TABLE.
  - NAM003: Warn on non-snake_case function names.
* Add test fixtures for all new rules.

testdata/lint/correctness_violations.sql

@@ -0,0 +1,10 @@
+-- Fixtures for correctness rules (COR001-COR003).
+
+-- COR001: SELECT *
+SELECT * FROM orders;
+
+-- COR002: UPDATE without WHERE
+UPDATE orders SET status = 'archived';
+
+-- COR003: DELETE without WHERE
+DELETE FROM orders;