feat(agent): rename InboxItemInfo to AgentMessage with an id; send returns it

Add a branded AgentMessageId assigned to each accepted send message and
returned from send/followup/steer/inject (was void). Rename the inbox
event payload InboxItemInfo to AgentMessage, carrying that id so a caller
can correlate a queued item with its enqueue/dequeue/discard events.

scripts/gen-cordis-catalog.ts

@@ -35,8 +35,9 @@ export const LINK_MAP: Record<string, string> = {
   ContinuationDecision: 'core.md',
   ContinuationStop: 'core.md',
   GenerateOptions: 'core.md',
+  AgentMessage: 'core.md',
+  AgentMessageId: 'core.md',
   HookContext: 'core.md',
-  InboxItemInfo: 'core.md',
   LlmCallConfig: 'core.md',
   LlmModelContext: 'core.md',
   LlmFailure: 'llm-streaming.md',