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/naming_violations.sql

@@ -0,0 +1,15 @@
+-- Fixtures for naming rules (NAM001-NAM003).
+-- Quoted identifiers preserve case in the PG parse tree; unquoted identifiers
+-- are always folded to lowercase by the parser and cannot violate snake_case.
+
+-- NAM001: quoted CamelCase table name
+CREATE TABLE "UserAccounts" (
+  -- NAM002: quoted camelCase column names
+  "userId"       integer PRIMARY KEY,
+  "emailAddress" text NOT NULL
+);
+
+-- NAM003: quoted CamelCase function name
+CREATE FUNCTION "GetUserById"(p_id integer) RETURNS text LANGUAGE sql AS $$
+  SELECT email FROM users WHERE id = p_id
+$$;