feat(mcp): books.mark_sent, and the send in the read tools (P4b S5)

`books.mark_sent` records the send the human or a delegate made: the
invoice (`invoice:7` or its id), the day, who sent it (the token's name
unless given), how (email by default, or mail, portal, hand, another
channel), to whom (the bill-to email, else the customer's), whether it
was the original or a reminder (original on the first send, reminder
after), and the channel's own id. It answers with every send on the
invoice, newest last. `books.documents` rows gain `last_sent` and
`books.document` shows the sends, so "did we send it, and when" is one
read away, while a bill (which we receive) has none.

The schema lint refuses a parameter named `to` (a path-name rule), so
the recipients parameter is `recipients`; the journal event and the
result keep the plan's `to`. The tool needs `books:write`: it changes
the record, never a figure.

The handbook gains the invoices-and-bills section (S4.5 had not landed
yet) with the send procedure: the books never send, the PDF goes to the
human, and each send is recorded afterwards.

Over the wire, mark_sent_records_who_and_how: two sends (the second a
reminder with a message id), the list's `last_sent`, the document's
`sends`, and the invoice still owing its total. The P2 cutover refuses
a send before consent and takes it after.

crates/ledger-mcp/handbook/invoices-and-bills.md

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+# Invoices and bills
+
+An invoice bills a customer: it posts Dr Accounts Receivable and Cr the
+revenue its lines name. A bill is a vendor's invoice to us: it posts Dr
+the expense or asset its lines name, and Cr Accounts Payable. A document's
+own figures are read from the ledger lines tagged with it, never kept on
+the document: a reversed payment, a void or a write-off comes out right
+without another copy to correct (B-38).
+
+## Sending an invoice
+
+- The books never send mail, and never say they did. To send an invoice,
+  create and post it (`books.create_invoice`), hand its PDF to the human
+  (`books.document` with `pdf: true`), and let the owner or a delegate
+  send it.
+- Then record the send with `books.mark_sent`: the invoice (`invoice:7`),
+  the day it went out, who sent it (`by`: the owner, or the agent acting
+  for them), how (`via`: `email`, or `mail`, `portal`, `hand`, or another
+  channel's own name), to whom (`to`: the bill-to email, else the
+  customer's email), whether it was the original or a reminder (`kind`:
+  `original` for the first send, `reminder` after), and the channel's own
+  reference when there is one (a message id).
+- A send changes no figure and never moves the due date. Several sends
+  are allowed: each is recorded in order, and `books.documents` shows the
+  latest as `last_sent`.
+- A void invoice can't be marked sent: it should never have reached the
+  customer. An invoice already open, partly paid, paid or written off can
+  (a copy for the customer's records, a reminder).