Add Weekly Update Household API feature

[hua7450]

Summary

Adds a GitHub Actions workflow for automated weekly updates of policyengine-us package.

Fixes #1178

Features

• Scheduled runs: Every Wednesday at 5 PM EST (same as policyengine-us)
• Manual trigger: Can be triggered via workflow_dispatch
• Auto-detection: Checks PyPI for latest policyengine-us version
• Changelog summary: Generates a summary of all changes between versions (Added/Changed/Fixed/Removed)
• Smart PR handling: Updates existing PR if one exists, otherwise creates new one
• Skip if no updates: Logs notice and exits if already at latest version

How It Works

1. Fetches current version from setup.py
2. Checks latest version on PyPI
3. If update available:
   • Updates setup.py with new version
   • Fetches changelog.yaml from policyengine-us repo
   • Extracts changes between old and new versions
   • Creates changelog_entry.yaml
   • Creates/updates PR with full changelog summary

Files Changed

• .github/workflows/weekly-update.yaml - New workflow file
• changelog_entry.yaml - Updated with feature addition

Test Plan

• Triggered workflow manually from branch
• Verified PR Weekly policyengine-us update #1180 was created with correct format
• Verified changelog summary generation works
• Remove temporary test triggers before merge (push trigger and ref override)