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_tax_policy.rs

@@ -278,7 +278,7 @@ fn require_tax_category_refuses_defect_8() {
     );
     let bill = m.call(
         "books.create_bill",
-        json!({"party_id": vendor["id"], "date": "2026-02-01", "due_date": "2026-03-01",
+        json!({"vendor": (vendor["id"]).to_string(), "date": "2026-02-01", "due_date": "2026-03-01",
                "vendor_ref": "CH-1",
                "lines": [{"description": "hosting", "amount": "42.00", "account": "6150"}],
                "upload_id": first_invoice, "confirmed": true}),
@@ -303,7 +303,7 @@ fn require_tax_category_refuses_defect_8() {
     );
     let again = m.call(
         "books.create_bill",
-        json!({"party_id": vendor["id"], "date": "2026-02-02", "due_date": "2026-03-02",
+        json!({"vendor": (vendor["id"]).to_string(), "date": "2026-02-02", "due_date": "2026-03-02",
                "vendor_ref": "CH-2",
                "lines": [{"description": "hosting", "amount": "1.00", "account": "6150"}],
                "upload_id": second_invoice, "confirmed": true}),