Bundles: sync to the PSA as one billable line with item-level detail retained (+ import existing PSA bundles)

Description: Salesbuildr bundles resolve to flat, individual line items when they sync to the PSA. The PSA has no concept that those lines belong together as a bundle, so it's impossible to bill a bundle as a single line while keeping the components for reporting, COGS, or contract detail — even though Salesbuildr's own quote view can already hide those components.

Use case: MSPs want to invoice a bundle as one line (e.g. "Managed Workplace") while retaining the underlying items internally. The current workaround — manually creating a proxy product with the bundle name and total price and zeroing the real components — works mechanically but requires a hand-maintained catalog product per bundle. Additionally, existing bundles built in the PSA can't be imported, so a company with a large PSA bundle library has to rebuild everything in Salesbuildr from scratch.

Proposed solution: Map Salesbuildr bundles onto the PSA's native bundle objects (Autotask Service Bundles, ConnectWise invoice groups on agreement additions, HaloPSA bundle lines): one visible parent line carrying the price, with component lines retained but suppressed/zero-priced. Also support importing existing PSA bundles into Salesbuildr.

Expected benefit: Clean single-line bundle invoicing with full item-level detail retained for reporting, and a much easier adoption path for companies with existing PSA bundle libraries.