feat(audit): books.audit, the attestation tools and the CPA gate (P8)

The whole re-audit through the tools: books.audit previews its checks, then
with the preview's digest runs them, files the workpaper and journals the
run; the same books, day and scope return the run that exists.
books.attest_receipt, books.review_entry and books.attest_period record a
named client's claim with its self-reported method; books.attestations
weighs each against policy.attestation_bar as it stands. Under
policy.cpa_package_requires_audit = refuse, books that declared audits
cannot export the CPA package without a counted audit for the handoff at the
current head unless a reason is given, which is journaled as a decision
(D-9). Needs attention asks for the month-end attestation, the annual audit
and the audit before the handoff. Rules B-45, the handbook section, the
interface, parity, receipt-store and testing records, and the As-built
block. Merge needs no go-ahead: the live books refuse the new writes until
the owner gives it.

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

crates/ledger-mcp/src/handbook.rs

@@ -87,6 +87,10 @@ pub const SECTIONS: &[(&str, &str)] = &[
     ),
     ("1099", include_str!("../handbook/1099.md")),
     ("cpa-handoff", include_str!("../handbook/cpa-handoff.md")),
+    (
+        "attestations-and-audit",
+        include_str!("../handbook/attestations-and-audit.md"),
+    ),
     (
         "settings-and-conventions",
         include_str!("../handbook/settings-and-conventions.md"),