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_violations.sql

@@ -0,0 +1,15 @@
+-- Fixtures for migration safety rules (MIG001-MIG003).
+-- Every statement here should trigger at least one finding.
+
+-- MIG001: non-concurrent index
+CREATE INDEX idx_orders_user ON orders(user_id);
+
+-- MIG002: ADD COLUMN NOT NULL without DEFAULT
+ALTER TABLE orders ADD COLUMN status text NOT NULL;
+
+-- MIG003: ADD FOREIGN KEY without NOT VALID
+ALTER TABLE orders
+  ADD CONSTRAINT fk_orders_user FOREIGN KEY (user_id) REFERENCES users(id);
+
+-- MIG003: ADD CHECK without NOT VALID
+ALTER TABLE orders ADD CONSTRAINT chk_positive CHECK (amount > 0);