feat: add structural reform support for AI agent

[nikhilwoodruff]

Adds ability for the AI agent to write and run structural reforms (simulation modifiers) on UK/US simulations.

Closes #66

Changes

Data model:

• Added simulation_modifier field to Policy model for storing Python code defining custom variable formulas
• Database migration to add the column

Modal functions:

• Updated _get_pe_policy_* functions to return tuple of (policy, simulation_modifier)
• Added _apply_simulation_modifier() helper to execute modifier code
• Updated all economy simulation/comparison functions to apply modifiers

Agent upload endpoints (/agent/results/*):

• POST /simulation - mark simulation completed/failed
• POST /household - upload household calculation results
• POST /decile-impact - upload decile impact data
• POST /program-statistics - upload program statistics
• POST /poverty - upload poverty metrics
• POST /inequality - upload inequality metrics
• POST /report-status - update report status
• POST /policy-with-modifier - create policy with simulation modifier

Agent tools:

• Added execute_python tool so the agent can test modifier code before submitting
• Runs Python in a sandboxed environment, returns stdout/stderr/exceptions

Agent system prompt:

• Added workflow for structural reforms with example modifier code
• Instructions to always test code with execute_python before submitting

Example usage

The agent workflow for structural reforms:

# 1. Test the modifier code first
execute_python(code="""
from numpy import where

def modify(simulation):
    pass

print(f"modify defined: {callable(modify)}")
# Test logic
income = 15000
benefit = 1000 if income < 20000 else 0
print(f"Test: income={income} -> benefit={benefit}")
""")

# 2. Create policy with modifier
POST /agent/results/policy-with-modifier
{
  "name": "New benefit for low income",
  "simulation_modifier": "def modify(simulation): ..."
}

# 3. Run economic impact analysis as normal
POST /analysis/economic-impact

Testing

Ran unit tests locally. Integration tests will run on CI.