fix(sqlite): enforce integer session metadata

docs/core-data-structures/persistence.md

@@ -53,7 +53,7 @@ interface SessionHeader {
   readonly version: number
   /** The session's id (mirrors the {@link Session}'s id). */
   readonly id: SessionId
-  /** Unix epoch milliseconds when the session was created. */
+  /** Non-negative safe-integer Unix epoch milliseconds when the session was created. */
   readonly createdAt: number
   /** Absolute working directory the session was created in (if any). */
   readonly cwd?: string