Update PolicyEngine UK to 2.88.11

[policyengine]

Summary

Update PolicyEngine UK from 2.88.0 to 2.88.11.

What changed (2.88.0 -> 2.88.11)

Changed

• • Update .github/CONTRIBUTING.md to document the towncrier changelog.d/ workflow. The old changelog_entry.yaml + make changelog flow was deprecated some time ago; the CONTRIBUTING guide still instructed new contributors to use it, causing CI round-trips on PRs that created a changelog_entry.yaml no fragment step was looking for.

Fixed

• • Fix gov.dwp.tax_credits.min_benefit parameter unit from currency-USD to currency-GBP — the parameter is a UK statutory threshold in pounds.
• Replace new_state_pension's flat-max payout with a min(reported, max) / max * period_max formula mirroring basic_state_pension, and extend additional_state_pension to NEW-type retirees so any pre-2016 SERPS/S2P Protected Payment flows through as an add-on instead of being silently dropped. Partial-NI-record retirees now receive their actual pro-rated rate rather than the full flat max. Closes part of the ~£12 bn residual state-pension gap vs the OBR target tracked in Update PolicyEngine US to 1.19.0 #1632.
• Zero out Income Support and income-based Jobseeker's Allowance after DWP managed migration completed on 31 March 2026. New parameters gov.dwp.income_support.active and gov.dwp.JSA.income.active flip to false from 2026-04-01, matching the Tax Credits treatment. Contribution-based JSA remains active.
• Bump policyengine-core minimum to >=3.25.0 to pick up the cache-invalidation and set_input preservation fixes (Preserve set_input values across apply_reform policyengine-core#475). The 3.24.0–3.24.3 cascade left UK model tests returning zero for income_tax, UC, and other formula-driven variables when a reform is applied during simulation construction; 3.25.0 includes the regression fix.
• Migrate versioning workflow to GitHub App token (POLICYENGINE_GITHUB PAT expired).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.66%. Comparing base (1690e20) to head (38a8968).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3542   +/-   ##
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  Coverage   76.66%   76.66%           
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  Files          63       63           
  Lines        3446     3446           
  Branches      621      621           
=======================================
  Hits         2642     2642           
  Misses        629      629           
  Partials      175      175           

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