feat: add delete-credit-note-allocation tool (+ show allocations in list-credit-notes)

[rathga]

What

Adds a delete-credit-note-allocation tool wrapping AccountingApi.deleteCreditNoteAllocations, and extends list-credit-notes output with each credit note's allocations (allocation ID, amount, date, target invoice) and remaining credit so allocation IDs are discoverable.

Why

create-credit-note-allocation can apply credit to an invoice, but there is currently no way to undo an allocation made in error — or to free up an allocated credit note so it can be corrected and re-applied. The common bookkeeping flow "de-allocate → fix → re-allocate" is impossible via the MCP today. Notably, once a credit note is fully allocated Xero flips its status to PAID, so nothing about it can be changed until the allocation is removed.

list-credit-notes previously didn't show allocations at all, so even with a delete tool there was no way to obtain an allocation ID; this PR surfaces them in the existing list output.

Changes

• src/handlers/delete-xero-credit-note-allocation.handler.ts — thin wrapper around deleteCreditNoteAllocations(tenantId, creditNoteID, allocationID)
• src/tools/delete/delete-credit-note-allocation.tool.ts — MCP tool (creditNoteId, allocationId)
• src/tools/delete/index.ts — register the tool
• src/tools/list/list-credit-notes.tool.ts — include remaining credit + allocations (with IDs) per credit note

Testing

• npm run build, npm run lint, npm test all pass
• Live-verified against a real Xero tenant: created a throwaway credit note + invoice, allocated the credit, confirmed the allocation (with its ID) appears in list-credit-notes, deleted the allocation via the new tool, and confirmed the allocation disappeared and the credit note's remaining credit returned to the full amount (status PAID → AUTHORISED). All fixtures voided/archived afterwards.

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