feat: Add custom field management and sequence enrollment

Summary

Add operations for managing custom fields and enrolling subscribers in sequences.

Proposed CLI Interface

# Custom fields
kit custom-field list
kit custom-field create --label "Company" --key "company"
kit subscriber set-field <id> --field "company" --value "Acme Corp"

# Sequence enrollment
kit sequence enroll <sequence-id> --subscriber-id <id>
kit sequence enroll <sequence-id> --tag "sales-ready"
kit sequence enroll <sequence-id> --email "user@example.com"

Kit API Endpoints

• List Custom Fields: GET /v4/custom_fields - API Docs
• Create Custom Field: POST /v4/custom_fields - API Docs
• Add to Sequence: POST /v4/sequences/{id}/subscribers - API Docs

Notes

This is a Phase 2 feature. Detailed implementation spec to be developed after Phase 1 analytics features are complete.

Key considerations:

• Custom field type validation
• Bulk enrollment options
• Handling subscribers already in sequence
• Respecting sequence entry rules (e.g., can't re-enroll completed subscribers)

Start with the proposed CLI commands and the linked Kit API documentation for custom fields and sequence enrollment. A detailed implementation specification is still needed, including validation, bulk enrollment, duplicate enrollment, and sequence entry rules; done means those decisions are reflected in working commands.