fix: required columns on newly created tables retain migration safety checks

DECISIONS.md

@@ -0,0 +1,12 @@
+# Architecture Decisions
+
+## Required columns on newly created tables
+
+**Decision.** The `NOT_NULL` SQL check accepts `ALTER TABLE ... ADD COLUMN ... NOT NULL`
+only when the same migration unconditionally created that exact table. `CREATE TABLE IF
+NOT EXISTS` does not prove an empty table. The migration remains
+fully linted, so `RunSQL`, `RunPython`, and operations on existing tables retain their
+checks.
+
+**Rejected.** Ignoring all initial migrations or naming them as exemptions. Either option
+skips unsafe operations that the linter must inspect.