refactor(mcp): the eight long tool descriptions move into the handbook (P5)

Agent reason: a tool description is read every time the tool list is, so
eight descriptions of 800 to 2,000 characters cost every session for text
only a few calls need. books.export, books.export_cpa_package,
books.import_bank_csv, books.reconcile, books.set_entry_party, books.set_w9,
books.show and books.vendor_1099 now say what the tool is for, what it
requires and where the rest is, in at most 700 characters; the rest is in
the handbook, verbatim: the CPA package's contents (cpa-handoff), the share
rules of a multi-party tag and the W-9 procedure (parties-and-tins), the
worksheet's construction (1099), the CSV import and the report list with
the IIF limits (files-in-out), the reconciliation's answer (reconciliation)
and the targets books.show opens (viewer).

tool_descriptions_fit_700_characters now pins zero and its allowance list is
deleted: no description may exceed the cap, and none is excused. The period
test keeps its sentence about omitted dates in books.export's description.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

crates/ledger-mcp/handbook/parties-and-tins.md

@@ -13,3 +13,15 @@
 - Never put a full TIN or SSN anywhere in the books, only the last four digits: the engine refuses any nine-digit number in a party field. The full TIN goes only to the encrypted TIN vault, and every member's and every 1099 payee's belongs there: books.set_tin (party, tin, tin_type: ssn, ein, itin; any role, so each member's SSN for their K-1) and books.set_w9 with tin and tin_type for a payee's W-9, and the signed form with w9_upload_id (books.begin_upload purpose w9; never books.file_document or books.attach_receipt: the form shows the TIN).
 - A full TIN lives in the vault: keep it out of descriptions, notes, file names and version-history messages. books.get_w9 hands the signed form over (run its download command to save it for the human); books.purge_tin (preview, then confirmed, with a reason) deletes a TIN no longer needed.
 - books.parties lists the directory (each party's TIN last four from its TIN or W-9 facts), books.party shows one party with the year's totals, and books.show party opens it in the viewer. Archive (books.update_party archived) parties no longer dealt with; never delete history.
+
+## Tagging an entry with several parties (books.set_entry_party)
+
+Pass party (an id or a name), or parties: [{party, amount? | percent?}] for several. Each share is an exact amount (cents), a percent (summing to 100; leftover cents to the first party), or neither (derived: a sole party gets all; members named together get what their capital or distributions lines say, else their company.members share). Once the entry is posted, a derived split among several parties is recorded as the exact amounts it resolves to, so a later change to company.members never re-splits posted history. Name both members on a joint member transaction (a reimbursement on 2100, a joint contribution) and let the shares derive; give amounts when a split is uneven.
+
+Refused: a party twice, mixed kinds of share, two parties with vendor records, and, once the entry has balanced lines, shares that do not add up to what it moved. A party with a vendor record also tags the entry's vendor; a party without one, or clearing, clears the vendor tag. Metadata only: amounts never change. An entry and its reversal carry the same parties (tagging either tags both), so each party's part nets to zero. Journaled as entry.party_set (one party) or entry.parties_set (parties). The result is the entry as books.get_entry gives it, each party's share resolved, with its direction and what of it was money or property in kind.
+
+## Recording a W-9 (books.set_w9)
+
+Record what a contractor's or landlord's W-9 says, or clear it (clear: true alone): the date received (received_on), line 3 entity type, line 4 exempt payee and FATCA codes, the TIN's last four digits, and where the form is kept. Give the full TIN as tin with tin_type (ssn, ein, itin) and it goes only to the encrypted TIN vault (its key kept apart from it, created on first use): the books keep its last four (tin_last4 is then optional, and refused if it disagrees) and a hash of the vault record, in the W-9 facts and the party's TIN facts alike (party.tin_set, as books.set_tin), and kept_at defaults to "wanderbooks vault". The CPA package carries the full TIN to the CPA.
+
+Give the signed form (a PDF, PNG or JPEG) as w9_upload_id (books.begin_upload purpose w9, then `wanderbooks upload`: a file never passes through you), and it goes to the vault too, never to the receipts and documents; read it back with books.get_w9. Never put a TIN in any other field, note, description or file: the engine refuses any nine-digit number, and no tool output echoes one. The party needs a 1099 role (contractor or landlord: books.set_party_roles). Record it before paying them. A party with a vendor record also gets its W-9 marked on file there (books.vendor_1099 missing_w9). Clearing W-9 facts whose TIN is in the vault needs books.purge_tin first. Journaled as party.w9_set. The result is the party (the TIN's last four only).