refactor(mcp): books.mark_sent in its own sent group (P4b S5)

S4.4 is working in tools/documents.rs, so the send tool moves beside it
rather than into it: `crates/ledger-mcp/src/tools/sent.rs`, declared like
every other group and named in docs/architecture.md's layout. The two
helpers the document read tools share (`sent_out`, `last_sent`) go to
tools/helpers.rs, which the groups share; the handler and its own
argument readers stay private to `sent`.

No tool name, schema or behavior changes: books.mark_sent is where it
was on the wire, only in a file of its own.

docs/architecture.md

@@ -149,6 +149,7 @@ groups are:
 
 - `documents` — invoices, bills, payments, voids, `books.documents`,
   `books.document`, the control check and the agings;
+- `sent` — `books.mark_sent`, the record that an invoice went out (P4b S5);
 - `entries` — `books.draft_entry`, `books.add_line`, `books.post_entry`,
   `books.reverse_entry`, `books.get_entry`, `books.search_entries`, the
   bank CSV import;