feat(messaging-ui): Inbox domain — unified work items + mail (contract + UI + mock)

Second domain in the SDK, mirroring messaging's adapter/provider/hooks/components:
- InboxAdapter (listInbox unified feed + transition + mailHistory/mailReply +
  optional directory/composeInternal/composeExternal), InboxProvider/useInboxAdapter
- hooks: useInbox (filter + transition), useMailThread (history + reply), useCompose
- <Inbox> (filter tabs, unified list, detail pane), <MailReader> (HTML in a
  sandboxed iframe + reply), compose modal (in-app / email); themeable styles
- MockInboxAdapter (seeded mentions/needs-reply/alert + folded mail threads +
  directory) shipped from ./adapters/mock-inbox
- 6 inbox tests (mock unify/transition/reply + render open/reply/compose); 64 green

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-messaging-ui/src/inbox/adapter.ts

@@ -0,0 +1,28 @@
+import type { InboxItem, InboxState, MailMessage, MailPerson } from './types';
+
+/**
+ * The inbox seam. A host implements this; the SDK renders it. Mirrors MessagingAdapter's philosophy:
+ * `listInbox` returns the UNIFIED feed (work items + mail folded in) so the folding logic lives in
+ * one place (the adapter, which knows both sources). Compose methods are optional and degrade.
+ */
+export interface InboxAdapter {
+  /** The unified inbox: work items + mail threads as rows, filtered by state (mail shows in OPEN). */
+  listInbox(state?: InboxState): Promise<InboxItem[]>;
+
+  /** Transition a work item's state (Done/Snooze/Archive…). Mail rows are not transitioned. */
+  transition(id: string, state: InboxState): Promise<void>;
+
+  /** Messages of one mail thread (HTML + text parts) for the reader. */
+  mailHistory(threadId: string): Promise<MailMessage[]>;
+
+  /** Reply into an existing mail thread. */
+  mailReply(threadId: string, content: string): Promise<void>;
+
+  // ── Compose (optional) — absent hides the "New message" affordance ──
+  /** People you can compose an in-app message to. */
+  directory?(): Promise<MailPerson[]>;
+  /** App-to-app mail (no SMTP) to a registered user's in-app inbox. */
+  composeInternal?(recipientUserId: string, subject: string, text: string): Promise<void>;
+  /** External email (SMTP) to an address. */
+  composeExternal?(target: string, subject: string, text: string): Promise<void>;
+}