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.

packages/ui/tui/tests/harness.ts

@@ -1,6 +1,7 @@
 import { Context } from 'cordis'
 import type { Terminal } from '@earendil-works/pi-tui'
 import AgentRegistry, {
+  AgentMessageId,
   type Agent,
   type AgentCancelCause,
   type AgentOptions,
@@ -152,16 +153,19 @@ export async function createTuiTestHarness<TerminalType extends Terminal, Exit e
     send(content, options) {
       sent.push(content)
       sentOptions.push(options)
+      return AgentMessageId('stub')
     },
     followup(content, options) {
       sent.push(content)
       sentOptions.push(options)
+      return AgentMessageId('stub')
     },
     steer(content, options) {
       steered.push(content)
       steeredOptions.push(options)
+      return AgentMessageId('stub')
     },
-    inject() {},
+    inject: () => AgentMessageId('stub'),
     cancel(cause = { kind: 'user' }) {
       cancelled.push(cause)
     },