Reconcile the Montana disability income subtraction

The legal code [Montana disability income exclusion rule 42.15.217](https://rules.mt.gov/gateway/RuleNo.asp?RN=42%2E15%2E217) does not mention the cap amount $5200, but the [tax form](https://mtrevenue.gov/wp-content/uploads/dlm_uploads/2022/12/Form-2-2022-Instructions.pdf#page=31) does.

Compare Montana rule 42.15.217 with the 2022 Form 2 instructions, page 31, and locate the implementation of the Montana disability income subtraction. Confirm which source governs the $5,200 cap, then update the relevant policy rule and add or adjust coverage so the subtraction matches the authoritative guidance.

Written by the indexing model from the issue text.