feat: allow date/reference updates on authorised credit notes

[rathga]

What

Relaxes the DRAFT-only guard on update-credit-note so that AUTHORISED credit notes can have their date and reference updated. Line items and contact remain locked once a credit note is approved, and PAID/VOIDED credit notes are still rejected.

Why

Xero's API (and UI) permit limited edits to an approved credit note — most commonly correcting its date or reference. The MCP currently rejects any update to a non-draft credit note, forcing users into the Xero UI for a simple date fix. This pairs with #194: a fully allocated credit note reports status PAID, so the workflow for correcting it is de-allocate (PAID → AUTHORISED) → fix the date via this tool → re-allocate.

Changes

• src/handlers/update-xero-credit-note.handler.ts — AUTHORISED credit notes accept date/reference updates; supplying lineItems or contactId for an authorised credit note returns a clear error; other non-draft statuses are rejected as before (with an updated message)
• src/tools/update/update-credit-note.tool.ts — tool description updated to match

Testing

• npm run build, npm run lint, npm test all pass
• Live-verified against a real Xero tenant: re-dated an AUTHORISED (unallocated) credit note successfully; confirmed the handler rejects a line-item update on an AUTHORISED credit note; confirmed a PAID (fully allocated) credit note is still rejected with the new error message. Throwaway fixtures voided/archived afterwards.

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