Add SQL diff tests

[jschatz1]

Problem

`kai-core/diff/sql.go` implements SQL-specific diffing logic, but the diff package test file only covers `diff.go` — there are no tests for SQL diffing or `format.go`.

What to Test

• SQL schema comparison (CREATE TABLE, ALTER TABLE)
• Column additions/removals
• Index changes
• Constraint changes
• Output formatting

Files to Look At

• `kai-core/diff/sql.go`
• `kai-core/diff/format.go`
• `kai-core/diff/diff_test.go` — existing tests to follow as a pattern