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.

docs/mcp-interface.md

@@ -2029,9 +2029,24 @@ S4 changes `books.create_invoice` and `books.create_bill`; see plan 8.2 S4.
   number and reads `void`. Refused while a payment stands on it (the
   refusal names the payment entry to reverse first), when void already,
   before its date or in a closed period (P4b S1, B-38).
+- `books.mark_sent { invoice, on?, by?, via?, recipients?: [string],
+  kind?, reference?, note? }` → `{ invoice, sends: [{ on, by, via, to, kind,
+  reference?, note? }] }`. Records that an invoice went out (P4b S5,
+  A-4): the books never send, so the owner or a delegate sends and this
+  records who (`by`: the token's name unless given), how (`via`: email
+  the default, or mail, portal, hand, another channel's name), to whom
+  (`recipients`: the invoice's bill-to email, else the customer's, else
+  required), when (`on`: today in the company's time zone), original or
+  reminder (`kind`: original on the first send, reminder after), and the
+  channel's own id (`reference`: a message id). `invoice` is
+  `invoice:N` or `N`. Several sends are allowed; none moves a figure or
+  the due date. Refused on a void invoice, dated before it, with no
+  recipients, or while the books wait for `documents_v2`
+  (`format_upgrade_pending`). `sends` is every send on the invoice, in
+  order, the new one last.
 - `books.documents { kind, status?, party?, due_by?, as_of? }` → `{ as_of,
   kind, documents: [{ ref, party, date, due_date, total, outstanding,
-  status, overdue_days }], outstanding_total }`. Invoices (`kind`
+  status, overdue_days, last_sent? }], outstanding_total }`. Invoices (`kind`
   `invoices`) or bills (`bills`), due date first, every figure derived
   from the document's lines that stand (B-38). `status` filters: `open`
   (default: anything still owed, partly paid and overdue included),
@@ -2039,11 +2054,14 @@ S4 changes `books.create_invoice` and `books.create_bill`; see plan 8.2 S4.
   own `status` is one of `open`, `partial`, `paid`, `written_off`,
   `void`, `overpaid` or `unattached` (`DocStatus::key()`). `party` (a
   customer's or vendor's name, ignoring case) and `due_by` (due on or
-  before that day) narrow the list (P4b S2).
+  before that day) narrow the list (P4b S2). `last_sent` is the day the
+  invoice last went out, when a send is recorded (P4b S5); absent for an
+  invoice never sent and for every bill.
 - `books.document { ref, as_of?, pdf? }` → `{ ref, party, date, due_date,
   entry_id, lines: [{ description, amount, project? }], total, paid,
   discounted, written_off, outstanding, status, overdue_days, timeline:
-  [{ date, entry_id, kind, amount, reversal, stands }], pdf? }`. `ref` is
+  [{ date, entry_id, kind, amount, reversal, stands }], sends: [{ on, by,
+  via, to, kind, reference?, note? }], pdf? }`. `ref` is
   `invoice:N` or `bill:N`; `status` as `books.documents`' rows; the
   timeline is every ledger line tagged with the document, dated on or
   before the day, in posting order, each saying whether its entry still