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

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+-- 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;

testdata/lint/migration_clean.sql

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+-- 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;

testdata/lint/migration_violations.sql

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+-- 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);

testdata/lint/naming_violations.sql

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+-- 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 $$
+  SELECT email FROM users WHERE id = p_id
+$$;