Fix individual_eitc reform not responding to parameter changes

[DTrim99]

Bug Description

The individual_eitc (Winship EITC) reform does not respond to parameter changes in the app. When setting enabled to true and agi_eitc_limit to 100000, there is no impact.

Root Cause

1. Reform activation check at creation time: The enabled parameter is checked in create_eitc_winship_reform() at reform creation time (line 5 of winship.py), not at simulation time. Since structural reforms are created with base parameters before user modifications are applied, the reform is never included when enabled defaults to false.

2. Missing takes_up_eitc multiplier: The reform's eitc formula omits the takes_up_eitc factor that exists in the baseline.

3. Default agi_eitc_limit of 0: When the limit is 0, the condition agi < 0 is false for virtually everyone.

Expected Behavior

When user sets enabled: true and agi_eitc_limit: 100000 in the app, households with AGI < $100,000 should receive the individual-income-based EITC calculation.

Solution

Restructure the reform to follow modern patterns (like NJ budget reforms):

• Check enabled inside the formula using where()
• Include takes_up_eitc multiplier
• Handle agi_eitc_limit = 0 as "no limit" rather than "no EITC"

Files to Modify

• policyengine_us/reforms/winship.py
• policyengine_us/parameters/gov/contrib/individual_eitc/agi_eitc_limit.yaml (update description)