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/migration_clean.sql

@@ -0,0 +1,17 @@
+-- Fixtures for migration safety rules — all statements are safe.
+
+-- MIG001 safe: CONCURRENTLY
+CREATE INDEX CONCURRENTLY idx_orders_user ON orders(user_id);
+
+-- MIG002 safe: nullable column (no NOT NULL)
+ALTER TABLE orders ADD COLUMN note text;
+
+-- MIG002 safe: NOT NULL with a DEFAULT
+ALTER TABLE orders ADD COLUMN status text NOT NULL DEFAULT 'pending';
+
+-- MIG003 safe: FK with NOT VALID
+ALTER TABLE orders
+  ADD CONSTRAINT fk_orders_user FOREIGN KEY (user_id) REFERENCES users(id) NOT VALID;
+
+-- MIG003 safe: CHECK with NOT VALID
+ALTER TABLE orders ADD CONSTRAINT chk_positive CHECK (amount > 0) NOT VALID;