feat(mcp): books.documents and books.document (P4b S2, read tools)

books.documents lists invoices or bills, due date first, with what each
still owes and its status, all derived from its ledger lines that stand
(B-38): status open (default), overdue, paid, void, written_off,
overpaid or all; party and due_by narrow it (what to pay this week).
books.document shows one (ref invoice:N or bill:N) as of a day: lines
with projects, total, paid, discounted, written off, outstanding,
status, days overdue, and its timeline of every tagged ledger line, each
saying whether its entry still stands. The over-the-wire test
documents_list_and_timeline_over_the_wire covers a reversed payment;
the happy-path journey covers both output schemas.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

docs/mcp-interface.md

@@ -2019,6 +2019,21 @@ accounting only: the caller supplies every figure), tax filing.
   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.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`
+  `invoices`) or bills (`bills`), due date first, every figure derived
+  from the document's lines that stand (B-38). `status` `open` (default:
+  anything still owed), `overdue`, `paid`, `void`, `written_off`,
+  `overpaid` or `all`; `party` (a customer's or vendor's name or id) and
+  `due_by` narrow the list (P4b S2).
+- `books.document { ref, as_of? }` → `{ 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 }] }`. `ref` is
+  `invoice:N` or `bill:N`; the timeline is every ledger line tagged with
+  the document in posting order, each saying whether its entry still
+  stands as of the day (P4b S2).
 - `books.control_check { as_of? }` → `{ as_of, agrees, accounts: [{ role,
   code, name, balance, documents_total, unattached: [{ entry_id, line,
   date, amount }], difference, attention: [{ kind, id, status, total,