fix: write-ahead audit entry so calls stop when the audit log stops accepting writes (SEC-03)

docs/architecture.md

@@ -115,8 +115,9 @@ One token verifier accepts either kind. The dispatcher turns the verified token
 
 ## Audit
 
-Every tool call, including failed and invalid ones, produces exactly one event in the
-append-only, hash-chained audit log. Only argument names are recorded, never values. Details and
+Every tool call, including failed and invalid ones, produces exactly one result event in the
+append-only, hash-chained audit log, after a write-ahead event that makes a call fail closed when
+the log stops accepting writes. Only argument names are recorded, never values. Details and
 integrity validation are in [audit-logging.md](audit-logging.md).
 
 ## Request flow
@@ -127,7 +128,9 @@ integrity validation are in [audit-logging.md](audit-logging.md).
    schema, and resolves the identity.
 4. The executor calls Gitea with the identity's token.
 5. The response is bounded, redacted and masked, and wrapped in the envelope.
-6. One audit event is written and the result is returned.
+6. One audit result event is written and the result is returned. (A write-ahead
+   `tool_call_started` event is written right after the tool lookup in step 3; when it cannot be
+   written the call is refused.)
 
 ## Module map