docs: describe the write-ahead audit model and bring the contract files up to date

docs/architecture.md

@@ -21,7 +21,7 @@ GiteaExecutor                         (gitea/executor.py)
 Gitea REST API (/api/v1)
 ```
 
-Every tool call ends in one audit event, and every result is wrapped in the same envelope.
+Every tool call is audited (a write-ahead `tool_call_started` entry, then one `tool_invocation` entry with the outcome), and every result is wrapped in the same envelope.
 
 ## Catalog