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/1099.md

@@ -5,3 +5,7 @@
 - Tag what members paid for the company with the card or the cash they used (books.set_entry_instrument): their card is excluded, their own check counts. A card that only funded a friends-and-family transfer is their cash. Tag what a payment platform's balance paid the same way: goods and services excluded, friends-and-family counts; untagged, both stay undetermined.
 - Card and payment-network payments are excluded (the processor's Form 1099-K), whoever's card it was. A bank payment with no method recorded counts: tag a debit-card payment with the card.
 - books.vendor_1099 is the year-end worksheet for the CPA. undetermined means "tell me how it was paid" or "decide this vendor". With include_tins: true it carries the full TINs from the vault; csv: true also hands it over as a CSV file (run its download command to save it).
+
+## How the worksheet is built (books.vendor_1099)
+
+Per vendor: what counts toward its 1099 (paid out of the cash accounts, or through a financing account with the method recorded, while not decided not_reportable; an entry with several parties counts only the vendor's share), what was paid by card or payment network (excluded: Form 1099-K's), what is undetermined (a member paid, method not recorded: books.set_entry_instrument), its 1099 decision, and its status (reportable at the threshold: $600 through 2025, $2,000 in 2026, indexed after; under_threshold; not_reportable; undetermined). It lists those payments, the W-9s missing, and each payee's TIN last four. include_tins adds full TINs from the vault; csv hands it over. cash_accounts and financing_accounts default to the settings' (as books.export_cpa_package's do); through stops the year at a day: given a package's accounts and its as_of, the CSV is the package's 1099 worksheet. A year whose threshold is not known is refused: pass threshold from the IRS. For the CPA to confirm.