Consolidate UC work allowance parameters (#289) (#1651)

Merge `work_allowance/with_housing.yaml` and
`work_allowance/without_housing.yaml` into a single
`work_allowance.yaml` with shared metadata (period, unit, uprating,
reference) propagated to children via `propagate_metadata_to_children`.

The parameter path
`gov.dwp.universal_credit.means_test.work_allowance.with_housing`
(and `...without_housing`) is unchanged — OpenFisca treats a file's
dict keys and subdirectory files equivalently. All 5 work_allowance
and uc_earned_income baseline tests still pass.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vahid-ahmadi

changelog.d/289.md

@@ -0,0 +1 @@
+- Consolidate UC work allowance parameters (`with_housing` / `without_housing`) into a single `work_allowance.yaml` file, with shared metadata propagated from the parent node.

policyengine_uk/parameters/gov/dwp/universal_credit/means_test/work_allowance.yaml

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+description: Universal Credit work allowance
+metadata:
+  label: Universal Credit work allowance
+  period: month
+  unit: currency-GBP
+  uprating: gov.benefit_uprating_cpi
+  propagate_metadata_to_children: true
+  reference:
+    - title: The Universal Credit Regulations 2013 reg. 22
+      href: https://www.legislation.gov.uk/uksi/2013/376/regulation/22
+with_housing:
+  description: Universal Credit work allowance if household receives housing support.
+  metadata:
+    label: Universal Credit work allowance with housing support
+  values:
+    # Skip 2013-2016, when the parameterization was different.
+    2016-04-11: 192
+    2018-04-09: 198
+    2019-04-08: 287
+    2020-04-06: 292
+    2021-04-12: 293
+    2021-11-24: 335
+    2022-04-06: 344
+    2023-04-01: 379
+    2024-04-01: 404
+    2025-04-01: 411
+without_housing:
+  description: Universal Credit work allowance if household does not receive housing support.
+  metadata:
+    label: Universal Credit work allowance without housing support
+  values:
+    # Skip 2013-2016, when the parameterization was different.
+    2016-04-11: 397
+    2018-04-09: 409
+    2019-04-08: 503
+    2020-04-06: 512
+    2021-04-12: 515
+    2021-11-24: 557
+    2022-04-06: 573
+    2023-04-01: 631
+    2024-04-01: 673
+    2025-04-01: 684