feat: add pgpm-ltree-helpers to system modules and PGPM_MODULE_MAP

[pyramation]

Summary

Registers the new @pgpm/ltree-helpers module in two places:

1. pgpm/export/src/export-utils.ts — Added ltree (native extension) and pgpm-ltree-helpers to DB_REQUIRED_EXTENSIONS. This ensures database exports include these as required extensions.

2. pgpm/core/src/modules/modules.ts — Added 'pgpm-ltree-helpers': '@pgpm/ltree-helpers' to PGPM_MODULE_MAP. This enables pgpm install to resolve the control name to the npm package.

Review & Testing Checklist for Human

• Verify ltree belongs in DB_REQUIRED_EXTENSIONS (it's already in constructive.control requires but was missing here)
• Confirm pgpm-ltree-helpers placement in the array ordering is acceptable

Notes

Companion PRs:

• pgpm-modules #64: upstream module source
• constructive-db #1007: downstream copy + constructive.control dependency

Link to Devin session: https://app.devin.ai/sessions/ffa3ed8652fc412f976accbdc229c88d
Requested by: @pyramation

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