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/package.json

@@ -18,6 +18,10 @@
       "types": "./dist/adapters/kernel-client.d.ts",
       "import": "./dist/adapters/kernel-client.js"
     },
+    "./adapters/mock-inbox": {
+      "types": "./dist/adapters/mock-inbox.d.ts",
+      "import": "./dist/adapters/mock-inbox.js"
+    },
     "./conformance": {
       "types": "./dist/conformance.d.ts",
       "import": "./dist/conformance.js"