feat(audit): fit the audit tools into the agent surface (P8)

After P5 merged first: a close leads to the audit and the period's
attestation, an audit run to its workpaper and the claims behind it, the
attest tools to the attestations and the CPA package; a refusal for want of
a standing audit is the guard audit_required with books.audit as its
remedy; the audit prompt runs the audit tools. policy.cpa_package_requires_audit,
unset, refuses once the books hold an audit run and warns before, so a
book that has not begun the programme is not stopped from its first handoff
by a rule it has not met; set to refuse, it holds from the start.
bookspec states B-45 (convention) in its checker.

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

crates/ledger-mcp/src/guards.rs

@@ -40,11 +40,14 @@ pub(crate) enum Guard {
     DuplicateVendorRef,
     /// The first invoice number cannot change once one is issued.
     FirstNumberLocked,
+    /// No audit stands at the books' current head to rest this on (plan
+    /// P8): run `books.audit`.
+    AuditRequired,
 }
 
 impl Guard {
     /// Every guard, for the registry test.
-    pub(crate) const ALL: [Guard; 10] = [
+    pub(crate) const ALL: [Guard; 11] = [
         Guard::StalePreview,
         Guard::FormatUpgradePending,
         Guard::ReceiptRequired,
@@ -55,6 +58,7 @@ impl Guard {
         Guard::UntaggedControlLine,
         Guard::DuplicateVendorRef,
         Guard::FirstNumberLocked,
+        Guard::AuditRequired,
     ];
 
     /// The code an agent reads.
@@ -70,6 +74,7 @@ impl Guard {
             Guard::UntaggedControlLine => "untagged_control_line",
             Guard::DuplicateVendorRef => "duplicate_vendor_ref",
             Guard::FirstNumberLocked => "first_number_locked",
+            Guard::AuditRequired => "audit_required",
         }
     }
 
@@ -91,6 +96,7 @@ impl Guard {
             Guard::UntaggedControlLine => "books.control_check",
             Guard::DuplicateVendorRef => "books.search_entries",
             Guard::FirstNumberLocked => "books.settings",
+            Guard::AuditRequired => "books.audit",
         }
         .to_string()
     }