fix: add frontend build step to generate-contacts workflow

[dbehnke]

Summary

Fixes the failed GitHub Actions workflow and changes it to create PRs instead of direct commits.

Problem

The generate-contacts.yml workflow was failing with:

main.go:22:12: pattern frontend/dist: no matching files found

This happened because main.go uses //go:embed frontend/dist to embed frontend static files, but the frontend/dist directory didn't exist in CI.

Changes

1. Added Frontend Build Step

• Installs Bun runtime
• Builds frontend to create frontend/dist before Go build

2. PR-Based Workflow (Instead of Direct Commit)

• Uses peter-evans/create-pull-request@v6 action
• Creates a new branch auto/update-contacts-{run_id}
• Opens PR with review checklist for generated contacts
• Auto-deletes branch after merge

3. Added Required Permissions

Benefits

• ✅ Reviewable generated files before merging
• ✅ Cleaner git history on main
• ✅ Easier to revert bad generations
• ✅ Traceable changes with PR numbers
• ✅ No pollution of main branch with auto-commits

Testing

After merging, you can test by:

1. Manually triggering the workflow, or
2. Pushing a change to any filters/*.csv file on main