refactor: replace Claude Code CLI with direct Claude API

Bypass the Claude Code CLI which has issues with MCP SSE in Modal containers.
Use the Anthropic SDK directly with a simple agentic loop.

- Define PolicyEngine tools (calculate_household, get_parameter, search_parameters)
- Implement tool execution with async job polling
- Clean agentic loop: call Claude → execute tools → loop until done
- Much simpler: ~250 lines vs complex PTY/subprocess handling

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

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

Author: nikhilwoodruff