feat(skills): add invoice-three-way-match skill for AP three-way matching

[Cloudz-319]

Summary

Adds an invoice-three-way-match skill that performs automated accounts payable three-way matching against purchase orders and goods receipts with GL coding.

Features

• Extracts and validates invoice data
• Three-way matches invoice line items against PO and goods receipt
• Applies general ledger coding by item category
• Flags discrepancies (price mismatches, quantity mismatches, unmapped categories)
• Produces structured payment approval proposal with exception list
• Fed by upstream invoice intake, feeds settle-invoice for payment execution

Harness

3 test cases covering:

• Full match (approved)
• Partial exceptions (conditional approval)
• Missing receipt with unmapped category (rejected)

All pass: runx harness .

References

Closes frantic bounty #51