Implement CPP retirement pension

- Add CPP retirement pension parameters (maximum, average, eligibility age)
- Implement eligibility based on age 60+ and contribution history
- Calculate benefit proportional to years of contribution (up to 40 years)
- Use average monthly benefit as basis for calculation
- Add test coverage for various age and contribution scenarios

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: MaxGhenis

policyengine_canada/parameters/gov/cra/benefits/cpp/retirement/average_monthly.yaml

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+description: Average monthly CPP retirement pension at age 65
+values:
+  2020-01-01: 710.41
+  2021-01-01: 702.77
+  2022-01-01: 727.61
+  2023-01-01: 758.32
+  2024-01-01: 816.52
+  2025-01-01: 844.53
+metadata:
+  unit: currency-CAD
+  period: month
+  label: CPP average monthly retirement pension
+  reference:
+    - title: CPP retirement pension amount
+      href: https://www.canada.ca/en/services/benefits/publicpensions/cpp/cpp-benefit/amount.html

policyengine_canada/parameters/gov/cra/benefits/cpp/retirement/eligibility_age.yaml

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+description: Minimum age for CPP retirement pension eligibility
+values:
+  2020-01-01: 60
+metadata:
+  unit: year
+  period: year
+  label: CPP retirement pension eligibility age
+  reference:
+    - title: CPP retirement pension eligibility
+      href: https://www.canada.ca/en/services/benefits/publicpensions/cpp/cpp-benefit/eligibility.html

policyengine_canada/parameters/gov/cra/benefits/cpp/retirement/maximum_monthly.yaml

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+description: Maximum monthly CPP retirement pension at age 65
+values:
+  2020-01-01: 1_175.83
+  2021-01-01: 1_203.75
+  2022-01-01: 1_253.59
+  2023-01-01: 1_306.57
+  2024-01-01: 1_364.60
+  2025-01-01: 1_433.00
+metadata:
+  unit: currency-CAD
+  period: month
+  label: CPP maximum monthly retirement pension
+  reference:
+    - title: CPP retirement pension amount
+      href: https://www.canada.ca/en/services/benefits/publicpensions/cpp/cpp-benefit/amount.html

policyengine_canada/tests/benefits/cpp/cpp_retirement_pension.yaml

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+- name: CPP retirement pension with full contributions
+  period: 2024
+  input:
+    age: 65
+    cpp_years_of_contribution: 40
+  output:
+    # Full contributions (40 years) = average monthly benefit
+    # $816.52 * 12 = $9,798.24
+    cpp_retirement_pension: 9_798.24
+
+- name: CPP retirement pension with partial contributions
+  period: 2024
+  input:
+    age: 65
+    cpp_years_of_contribution: 20
+  output:
+    # 20/40 years = 50% of average benefit
+    # $816.52 * 0.5 * 12 = $4,899.12
+    cpp_retirement_pension: 4_899.12
+
+- name: No pension below age 60
+  period: 2024
+  input:
+    age: 55
+    cpp_years_of_contribution: 30
+  output:
+    cpp_retirement_pension: 0
+
+- name: No pension without contributions
+  period: 2024
+  input:
+    age: 65
+    cpp_years_of_contribution: 0
+  output:
+    cpp_retirement_pension: 0
+
+- name: CPP retirement pension for 2025
+  period: 2025
+  input:
+    age: 70
+    cpp_years_of_contribution: 45
+  output:
+    # Capped at 40 years = full average benefit
+    # $844.53 * 12 = $10,134.36
+    cpp_retirement_pension: 10_134.36
+
+- name: Minimum contribution scenario
+  period: 2024
+  input:
+    age: 60
+    cpp_years_of_contribution: 1
+  output:
+    # 1/40 years = 2.5% of average benefit
+    # $816.52 * 0.025 * 12 = $244.956
+    cpp_retirement_pension: 244.956

policyengine_canada/variables/gov/cra/benefits/cpp/cpp_retirement_eligible.py

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+from policyengine_canada.model_api import *
+
+
+class cpp_retirement_eligible(Variable):
+    value_type = bool
+    entity = Person
+    label = "Eligible for CPP retirement pension"
+    definition_period = YEAR
+    documentation = "Whether person is eligible for CPP retirement pension"
+    
+    def formula(person, period, parameters):
+        p = parameters(period).gov.cra.benefits.cpp.retirement
+        
+        # Check age requirement (60+)
+        age = person("age", period)
+        meets_age = age >= p.eligibility_age
+        
+        # Check contribution requirement (simplified - at least 1 year)
+        years_contributed = person("cpp_years_of_contribution", period)
+        has_contributions = years_contributed > 0
+        
+        return meets_age & has_contributions

policyengine_canada/variables/gov/cra/benefits/cpp/cpp_retirement_pension.py

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+from policyengine_canada.model_api import *
+
+
+class cpp_retirement_pension(Variable):
+    value_type = float
+    entity = Person
+    label = "CPP retirement pension"
+    definition_period = YEAR
+    unit = CAD
+    documentation = "Annual CPP retirement pension amount"
+    adds = ["cpp_retirement_pension"]
+    
+    def formula(person, period, parameters):
+        p = parameters(period).gov.cra.benefits.cpp.retirement
+        
+        # Check eligibility
+        eligible = person("cpp_retirement_eligible", period)
+        
+        # Simplified calculation based on years of contribution
+        # Use average between 0 and maximum based on contribution years
+        years_contributed = person("cpp_years_of_contribution", period)
+        
+        # Assume full contributions after 40 years
+        contribution_factor = min_(years_contributed / 40, 1)
+        
+        # Calculate monthly amount between 0 and maximum
+        # For simplicity, use average as midpoint
+        monthly_amount = contribution_factor * p.average_monthly
+        
+        # Convert to annual
+        annual_amount = monthly_amount * 12
+        
+        return where(eligible, annual_amount, 0)

policyengine_canada/variables/gov/cra/benefits/cpp/cpp_years_of_contribution.py

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+from policyengine_canada.model_api import *
+
+
+class cpp_years_of_contribution(Variable):
+    value_type = float
+    entity = Person
+    label = "Years of CPP contributions"
+    definition_period = YEAR
+    unit = "year"
+    documentation = "Number of years person has contributed to CPP"