feat(acp): the bridge approval answerer + scripted permission answers

The ACP bridge registers the first real approval answerer: an ask for an
agent it owns becomes session/request_permission attached to the already-
streamed tool call (one-shot allow_once/reject_once only), outcomes map
conservatively (unknown optionId never grants, client cancel → cancelled),
and foreign or call-less requests delegate down the waterfall. The snapshot
harness accepts scripted permissionAnswers (FIFO; an unscripted prompt
answers cancelled, fail closed) so recorded scenarios can drive the wire
keylessly.

scripts/gen-doc-graphs.ts

@@ -164,7 +164,7 @@ const SERVICE_ROLES: ServiceRole[] = [
     pkg: 'approval',
     title: 'Approval seam',
     mode: 'seam',
-    implementations: [],
+    implementations: ['acp'],
     consumers: ['tools'],
     note: 'One-shot permission decisions dispatched over the `approval/request` waterfall; answerers are listeners (the ACP bridge for its own agents), absence fails closed to `unavailable`.',
   },