Document council_tax_less_benefit as a public-facing convenience (#1671) (#1694)

Audit per #1671: the variable is unused inside policyengine_uk/ but is
consumed by several sibling repos (policyengine-app countries.js,
council-tax-ctr-map backend, policyengine-uk-data calibration targets, and
structural-reforms-demo). Keep it, and make its public-facing role explicit
in the docstring so future audits don't flag it again.

Author: vahid-ahmadi

changelog.d/1671.md

@@ -0,0 +1 @@
+- Document `council_tax_less_benefit` as a public-facing convenience variable used by downstream consumers (`policyengine-app`, `council-tax-ctr-map`, `policyengine-uk-data` calibration targets) rather than the core model itself, so future audits do not flag it as dead code.

policyengine_uk/variables/household/consumption/council_tax_less_benefit.py

@@ -3,7 +3,13 @@
 
 class council_tax_less_benefit(Variable):
     label = "Council Tax (less CTB)"
-    documentation = "Council Tax minus the Council Tax Benefit"
+    documentation = (
+        "Council Tax minus Council Tax Benefit / CTR. Kept as a public "
+        "convenience variable for downstream consumers (e.g. policyengine-app, "
+        "council-tax-ctr-map, policyengine-uk-data calibration targets) that "
+        "need the net council tax bill on a single line; not consumed inside "
+        "the core model itself."
+    )
     entity = Household
     definition_period = YEAR
     value_type = float