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/tools/audit.rs

@@ -683,7 +683,15 @@ impl BooksServer {
             .find(|q| q.name == named || q.id.to_string() == named)
             .map(|q| q.id)
             .ok_or_else(|| format!("no period {named:?}: books.list_periods lists them"))?;
-        let (a, _) = store.attest_period(period, claim).map_err(err)?;
+        let (a, _) = store.attest_period(period, claim).map_err(|e| match e {
+            ledger_store::StoreError::AuditRefused(why) => serde_json::json!({
+                "code": "audit_required",
+                "message": why,
+                "remedy_tool": "books.audit",
+            })
+            .to_string(),
+            other => other.to_string(),
+        })?;
         Ok(Json(attest_out(&store, &a)?))
     }