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.
swaits
adc8804f61