Add wp ppom export/import WP-CLI commands for site migration

Problem

PPOM stores field configurations in a custom nm_personalized table that standard WP migration tools may not export. Users deactivating during site migration lose their PPOM configurations. Two users in one batch left specifically during migration (candlemakers.co.za, kwaaiplanks.co.za).

Solution

New PPOM_CLI_Command class registered as wp ppom:

Export:
wp ppom export [--file=<path>]

• Exports all field groups from nm_personalized table to JSON
• Includes product assignments (_product_meta_id post meta)
• Records plugin version, export date, and source site URL

Import:
wp ppom import <file> [--skip-assignments]

• Imports field groups from JSON with auto-increment ID remapping
• Re-assigns to products with remapped IDs
• --skip-assignments flag for when product IDs differ between environments

Files Affected

• New: classes/cli.class.php — WP-CLI command class
• woocommerce-product-addon.php — conditional require when WP_CLI is defined

Acceptance Criteria

• wp ppom export creates valid JSON with field groups + assignments
• wp ppom import restores field groups with new auto-increment IDs
• Product assignments remapped to new IDs
• --skip-assignments flag skips product reassignment
• Commands only load when WP_CLI is defined
• Migration guide documentation published

Priority: Low — affects small number of users during migration
Regression risk: Isolated — CLI-only, no web impact

Start in woocommerce-product-addon.php to trace conditional loading when WP_CLI is defined, then inspect the nm_personalized table and _product_meta_id assignments described in the issue. Use classes/cli.class.php as the implementation entry point and verify export/import, ID remapping, skip behavior, conditional loading, and migration-guide documentation against the acceptance criteria.