refactor(mcp): the party_id, document_id and cash_account aliases are removed (P5)

Agent reason: one name for one thing. S4 and S6 kept the old spellings "until
P5" so an agent that learned them did not break; P5 removes them, so the
schema an agent reads has one parameter for each fact and a call cannot name
the same thing twice with different answers.

books.create_invoice takes customer and books.create_bill vendor (a record's
id, its party's id or its name; required), no party_id.
books.receive_payment and books.pay_bill take invoice or bill and
deposit_to or paid_from, no document_id or cash_account; a payment names its
document and its account or is refused saying which to pass. A number that is
one record's id and another's name is still refused, never guessed (INVARIANT
6).

The over-the-wire tests that used the old spellings now use the new
(document_id becomes invoice or bill, cash_account becomes deposit_to or
paid_from, party_id becomes customer or vendor given as the id's text); the
one test that named the alias, party_id_alias_still_works, is now
a_customer_by_id_and_by_name_is_one_record and checks the same two spellings
of a customer, by id and by name, name one record.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

crates/ledger-cli/tests/spec_attachments.rs

@@ -424,7 +424,7 @@ fn a_file_links_to_every_kind_of_subject() {
     let rent_invoice = m.upload(b"rent invoice", "rent.pdf", "application/pdf", "receipt");
     let bill = m.ok(
         "books.create_bill",
-        json!({"party_id": vendor, "date": "2026-02-01", "due_date": "2026-02-28",
+        json!({"vendor": (vendor).to_string(), "date": "2026-02-01", "due_date": "2026-02-28",
                "vendor_ref": "RENT-2",
                "lines": [{"description": "Rent", "amount": "300.00", "account": "6000"}],
                "upload_id": rent_invoice, "confirmed": true}),
@@ -432,7 +432,7 @@ fn a_file_links_to_every_kind_of_subject() {
         .clone();
     let invoice = m.ok(
         "books.create_invoice",
-        json!({"party_id": customer, "date": "2026-02-01", "due_date": "2026-03-01",
+        json!({"customer": (customer).to_string(), "date": "2026-02-01", "due_date": "2026-03-01",
                "lines": [{"description": "Work", "amount": "500.00"}],
                "confirmed": true}),
     )["id"]